John Mackey, chairman and chief executive of Whole Foods Market Inc. is the latest to learn that lesson, after he was revealed this week as the author, under a pseudonym, of pro-Whole Foods comments on an Internet stock-message board. Mr. Mackey also maintains a blog that has attracted the attention of federal antitrust regulators.

More CEOs and other top executives are venturing online, most notably with signed blogs, or personal Web pages on which they opine openly. Blogs offer CEOs a way to communicate with investors, employees and customers, and to defend themselves when under fire. Top-level corporate bloggers include Jonathan Schwartz, CEO of Sun Microsystems Inc., J.W. "Bill" Marriott Jr., chairman and CEO of Marriott International Inc., Michael Critelli, executive chairman of Pitney Bowes Inc., and Robert Lutz, vice chairman of General Motors Corp.

Done well, "a blog is an extremely effective way of articulating a vision," says Debbie Weil, a corporate blogging consultant and author of "The Corporate Blogging Book."

But blogs, chat rooms and stock-message boards also pose pitfalls, particularly for executives unaccustomed to unfiltered dialogue. Some CEOs deliberately stay away from newsy or controversial subjects in their posts (Mr. Marriott remarks on his blog, "It's really exciting to open a new hotel") and many companies now review executive writings before they are posted online.

Some executives comfortable in the online world questioned the wisdom of Mr. Mackey's actions. "It's a huge mistake for a CEO of a public company to post under an undisclosed handle," says Mark Cuban, the billionaire majority owner of professional basketball's Dallas Mavericks, president of HDNet and an avid blogger himself. He began his own blog in March 2004 as a response to a reporter who Mr. Cuban thought had taken his answers out of context. His first entry consisted of posting their entire email exchange.

Last month, Mr. Cuban wrote about his colonoscopy: "One minute I'm talking rugby, the next I'm waking up, picking up the conversation where I left off and being told to 'dispell the air in my system,'"

Because he doesn't work for a public company, "no one monitors or screens my work," Mr. Cuban wrote in an email interview. "I have unlimited space to say exactly what is on my mind and no timetable but my own in which to say it."

The CEO of job site Jobster Inc. attracted criticism from other bloggers last year after writing online about his iPod playlist as rumors swirled that the company was planning layoffs. Giant Wal-Mart Stores Inc. was embarrassed after a public-relations agency set up a pro-Wal-Mart blog but didn't fully reveal the identities of the bloggers. Countless executives have been haunted by ill-considered emails.

Mr. Mackey's postings under a false name were very different from a signed blog entry. While many bloggers yesterday criticized his behavior, legal experts yesterday said it was unclear whether he had violated securities law by touting Whole Foods' stock and denigrating that of Wild Oats Markets Inc., a rival that Whole Foods now wants to buy.

It appears from his voluminous postings that at times Mr. Mackey made financial predictions that weren't readily available from company disclosures to the markets.

At the 2006 annual meeting, Mr. Mackey told shareholders the company would hit $12 billion in sales by 2010, doubling its sales in five years. Less than a week later, under the pseudonym "harodeb," he was even more confident in an online posting: "The upgraded prediction of $12 billion is most likely conservative. Won't surprise me if the number ends up close to $14 billion in 5 years."

Later the same month, defending himself against a charge that he was a stock "pumper," harodeb predicted that "operating cash flow for 2006 will be up at least another 20 percent just as it is every year." Despite his rosy prediction, operating cash flow in fiscal 2006 rose only 10.2 percent. A Whole Foods spokeswoman declined to comment on both postings.

Whether they are unlawful could depend on several factors, including whether the statements were misleading or intended to manipulate the price of the stock. Whole Foods also could argue that the Yahoo message board is freely open to the public. An SEC spokesman declined to comment.

So far, there appear to be few other consequences. A big Whole Foods investor said it was sticking by Mr. Mackey, and, as of midafternoon the Whole Foods board hadn't met, according to a company spokeswoman. She declined to say whether any meetings were planned. Mr. Mackey declined an interview request yesterday. Whole Foods shares fell 50 cents, or 1 percent, to $39 in heavy trading on the Nasdaq.

Still, public-relations experts said Mr. Mackey's postings risked damaging Whole Foods' well-regarded brand. "Their trust bank is probably sufficient enough to weather this storm, but it chips away at some of their golden brand, because they go from being the company that cares about our health to just another company," said Richard S. Levick, a lawyer who runs Levick Strategic Communications, a crisis-communications firm.

Mr. Mackey's choice of forum was unusual as well. Stock-message boards were popular for a time during the tech-stock boom in the late 1990s, but fell out of favor after reports that investors had used the boards to circulate rumors that moved share prices. Most posts are anonymous, and few contain verifiable information. "It's the Wild West if you're going to use that stuff," says Todd Clark, director of stock trading at Nollenberger Capital Partners Inc. in San Francisco.

Mr. Mackey's postings recalled a 1999 incident involving mutual-fund giant Franklin Resources Inc. William Johnson, the son of then-CEO Charles Johnson, used a pseudonym (FlmMker9899) to defend the company and his father on an online message board. William Johnson wasn't a Franklin employee. Franklin Resources "has a longstanding internal policy that applies to all employees regarding public discussion in various public forums," spokeswoman Stacey Johnston said yesterday.

Forty of the country's biggest 500 companies now publish corporate blogs, according to the Fortune 500 Blogging Wiki, a collaborative tracking site. Technology companies such as Cisco Systems Inc., Oracle Corp. and Amazon.com Inc. embraced the idea early, but senior executives at industrial giants like Boeing Co. and GM have adopted the trend.

Mr. Critelli at Pitney Bowes started blogging last month. One post asserted that mail is more environmentally friendly than it gets credit for. "That's a very difficult concept to capture in a sound bite" but the blog allows him to explain it fully, he says.

The company's public-relations and legal teams review his posts, but have suggested only minor changes, he says. He likes the relative freedom of the blog, which notes that the views are his own. When he gave a speech as CEO, Mr. Critelli says, there were "multiple sign-offs and heavy edits." Now, "I tell them I welcome that feedback but not to hold it up."

Mr. Lutz, of GM, launched his FastLane blog in early 2005. He often writes about new products. Each proposed posting "is looked at by a number of eyes before it goes out to make sure it's accurate and we're not getting ahead of ourselves,''" says Steve Harris, the auto maker's vice president of public relations. In most cases, he says, "it goes through the way Lutz did it.''"

Sun, which boasts about 3,500 employee blogs, publishes internal guidelines, with warnings such as, "Using your weblog to trash or embarrass the company, our customers or your co-workers is not only dangerous but stupid.''" Sun's lawyers have told him not to disclose financial results or similar information solely in the blog.

Mr. Schwartz has been pushing the Securities and Exchange Commission to allow companies to post significant financial information on the Internet as a way to meet fair-disclosure rules. Last fall, Mr. Schwartz posted on his blog a copy of a letter on the subject that he sent to SEC Chairman Christopher Cox. Mr. Cox blogged back (and also sent a letter) applauding the idea but also raising some concerns.

Mr. Marriott, 75, is perhaps the most unusual corporate blogger. He doesn't use a computer. Instead, he records his musings on a digital recorder, which is then transcribed and posted to the Internet by a Marriott employee.

His blog discusses the hotel industry, but is also highly personal. Entries recount the birth of his granddaughter, memories of his parents' root-beer stand, and thoughts on immigration reform.

-- Tamara Audi, Russell Gold, Justin Lahart and Kara Scannell contributed to this article.