But only days ago, it seemed inconceivable that Mr. Gregory, who has run all of the firm’s major capital markets divisions, would lose his job. He and Mr. Fuld are so close that they often wander in and out of each other’s offices, discussing both family and business. Last Saturday, as Lehman scrambled to drum up new investors, the pair had dinner together with their wives.

This week, though, Lehman’s stock plunged in a deluge of selling, despite the capital raising, which one person close to the firm said was over-subscribed. Still, several analysts downgraded the company, and it became clear that a drastic step was necessary.

On Wednesday afternoon, Mr. Gregory and Mr. Fuld discussed their options. Shortly before the market closed at 4 p.m., the two decided Mr. Gregory would resign, said a person close to the situation. He will remain at Lehman in an unspecified role.

Ms. Callan, feeling the pressure, resigned a few hours later, that person said. She will return to work in Lehman’s investment banking division.

It is a remarkable turnabout for Ms. Callan, who, unlike Mr. Gregory, played no significant role in Lehman’s mortgage business. A tax lawyer by training, she stepped into a storm when she became chief financial officer in December. For months, she tried to dispel rumors that Lehman might run into trouble like Bear Stearns. In recent weeks, she struggled to rebut criticism of Lehman by David Einhorn, a hedge fund manager who is betting against Lehman stock.

Now, it will fall to Mr. Fuld to persuade both investors and employees that Lehman is sound. He plans to meet with the managing directors in New York and London next week to try to improve morale. Lehman may yet sell part of itself to a strategic investor in South Korea, which would most likely buy the stake in the open market, said a person close to the talks.

On Thursday, Mr. Lowitt and Mr. McDade, known as Bart, received a standing ovation when they addressed employees on Lehman’s trading floor. Mr. Fuld tried to rally his troops with a speech over the bank’s internal speaker system.