Scholars, rejoice. Starting December 5, the New York Public Library’s flagship research library is opening on Sundays for the first time in 34 years.

At a time when two counties in the country, Erie County in upstate New York and Salinas County in California, are planning to close their public libraries, the New York Public Library is also extending its hours at nearly half of its 85 branches in the Bronx, Manhattan, and Staten Island.

The news came yesterday at the annual meeting of the library’s board of trustees. The board also voted in a new chairwoman and mapped out a series of initiatives to improve the library.

“The New York Public Library is the foremost public library in the country. Everything we do sends a signal to other libraries,” said the board’s new leader, Catherine Marron.

At the meeting, the library’s president, Paul LeClerc, also announced a $25 million grant – the largest individual gift in the library’s history. It is being devoted to an initiative to acquire, preserve, and improve access to research materials in digital format. The donor is a philanthropist who made his fortune on Wall Street as a money manager, Robert Wilson.

In expanding its hours of service, the New York Public Library said its Humanities and Social Sciences Library, on Fifth Avenue at 42nd Street, will be open between 1 and 6 p.m. on Sundays.

In addition, 39 branches are adding between one and three hours, generally in the mornings, starting December 6, with the aim of accommodating mothers with small children and senior citizens. In January, February, and March, the library plans to start adding a sixth day of service at 14 branches that currently are open only five days a week.

The additional hours, as well the acquisition of new books and the enhancement of English as a Second Language programs, were made possible by enhancements in the library’s budget passed this summer by the City Council. The budget enhancements have also resulted in increased hours at the Brooklyn Public Library and the Queensborough Public Library.

But the longer hours represent merely a restoration of services the libraries once offered. “Unfortunately, we’re trying to go back to where we were. The demand is there, the demand has always been there,” said the director and chief executive of the branch libraries, Susan Kent.

“None of us, including the New York Public Library, are open the number of hours that we were just a few short years ago, and certainly not the hours that we know the people of the city need and want,” said the executive director of the Brooklyn Public Library, Ginnie Cooper.

In addition to the historic grant from Mr. Wilson, the library announced that it has received a $5 million grant from the Andrew W. Mellon Foundation to support the operations of the research libraries in the next three years. The money is to be used to implement cost saving measures and buffer the impact of a decision to reduce the amount of endowment interest the library spends.

“As government support of libraries is cut back, libraries are getting more aggressive at going after private support,” said the editor of the Chronicle of Philanthropy, Stacy Palmer.

The New York Public Library occupies a unique position among American libraries: supporting both a branch system and a research library. It the only public library to make the “Philanthropy 400,” a list of the nation’s largest nonprofit institutions, which is published by the Chronicle of Philanthropy.

A past president of the American Library Association, Maurice Freedman, lamented the situation for other libraries.

“It varies from community to community, how well or poorly libraries are funded,” he said, “but it’s tragic that there are some communities so bereft of resources that it look to close their public libraries – the single most important free institution that serves everybody in the community.”

Mrs. Marron praised New Yorkers for supporting the library, as well as the “ingenuity and thoughtfulness of the library’s outstanding leadership,” including new staff members such as David Offensend, the chief financial and administrative officer, who once worked down the hall from Mrs. Marron at Lehman Brothers.

Mrs. Marron is the second woman, after Elizabeth Rohatyn, to serve as a chairperson. A Wellesley graduate and former investment banker, she is a contributing editor at Vogue. Her husband, Donald, the chief executive of Paine Webber before it merged with UBS, is a past president of the board of the Museum of Modern Art, where Mr. Marron cut the ribbon yesterday on the Donald B. and Catherine C. Marron Atrium.

Mrs. Marron said she and her husband are lucky to be in a position to support their passions.

“I so deeply love this library, it’s in my bones,” she said. “I feel so deeply about its mission to bring the collections, services, programs, technology, and Internet access to the public, free of charge. It is essential to a democracy.”