Dechert L.L.P., one of the nation's largest and most profitable law firms, announced Thursday that its chairman and chief executive officer, Barton J. Winokur, will step down next year and be replaced as chairman by Andrew J. Levander, a top white-collar defense lawyer based in New York.

The firm said that mergers-and-acquisitions lawyer G. Daniel O'Donnell will take over day-to-day operations as CEO, managing Dechert's 19 offices in the United States and abroad, when Winokur steps down in mid-2011.

Winokur, a corporate and transactions lawyer who spent his entire career at the firm but for a one-year clerkship at the U.S. Court of Appeals for the Third Circuit after graduation from Harvard Law School, took over as chairman in 1996.

Since then, per-partner profit has increased from $345,000 to about $2 million, making Dechert one of the most financially successful firms in the country.

But, after years of sharply spiking profit and surging international growth, the firm also was hit hard by the financial collapse of 2008 and 2009, as were many other law firms.

Dechert's structured-finance practice, based in New York, saw a particularly steep drop-off, forcing at least two firmwide rounds of layoffs of lawyers and staff.

Those economic shocks caused revenue to decline 8 percent last year. Lawyer head count declined from a high of about 1,000 before the onset of the recession to just more than 800 currently.

The Dechert announcement was the second in the last month that a large Philadelphia-based firm had made a leadership change. In early May, Ballard Spahr L.L.P. said that longtime chairman Arthur Makadon would step down next year to be replaced by litigator Mark Stewart.

Winokur said in an interview Thursday that the transition had been in the works for some time, that he welcomed the change, and that he planned to return to the full-time practice of law.

"I'd been doing it for 14 years; one of the things that you need to do is assure your partners that there is a succession plan in place," Winokur said. "Without a succession plan, people worry about continuity."

He added that new leadership would bring a fresh perspective.

"People bring new energy when they start," he said.

A key challenge for retiring law-firm leaders often is how to rebuild a client base after years of focusing on internal management.

Winokur, a resident of Bryn Mawr who was raised in the West Oak Lane and West Mount Airy sections of the city, likely will have no such problem, since he devoted substantial time to client matters while running the firm.

He is known for prodigious productivity and typically has spent 800 hours a year or more representing clients, even as he managed Dechert's complex, global operations.

One of his recent high-profile representations was of the Haas Family Trusts during its negotiations with Dow Chemical Co. in the acquisition of Rohm & Haas Co.

His successor as chairman, Levander, is a relative newcomer to Dechert, joining the firm in 2005. Law firms typically choose their top fee-getters as leaders and Levander would fit that pattern.

He has one of the nation's most robust white-collar defense practices, defending the board of directors of Lehman Bros. Holdings Inc. after the collapse of the firm, and J. Ezra Merkin, the head of one of the so-called feeder funds that channeled investor money to Bernard L. Madoff, who pleaded guilty to running a massive Ponzi scheme that cost investors billions.

For the last several years, big law firms such as Dechert have faced sharp criticism from their clients over costs, most notably starting salaries for first-year lawyers that hit $165,000 in some cities. That pressure intensified in 2008, as the economy went into a deep recession, and corporate legal budgets became stretched.

Levander, a former assistant U.S. attorney in Manhattan, said that the challenges posed by the increasingly cost-conscious legal departments of big clients likely would persist and that the firm would focus on providing quality legal services while keeping costs under control.

"We try and look at what value we bring to clients; we are very aware of their budgetary needs," Levander said.