The Smithsonian Institution plans to announce today that it is receiving a $45 million donation for the Smithsonian American Art Museum and the National Portrait Gallery, making the donor -- the Donald W. Reynolds Foundation -- the second-largest benefactor to the museum in the past dozen years.

A spokesman for the Smithsonian declined to identify the donor but confirmed the $45 million figure. A source within the Smithsonian with knowledge of the gift confirmed that it is from the Reynolds foundation. The source requested anonymity so as not to be seen as preempting today's announcement.

Christian W. Kolberg, the foundation's director of communications, said yesterday, "I can neither confirm nor deny that."

The charitable group, based in Las Vegas, gave $30 million to the Smithsonian in 2001. That gift enabled the Smithsonian to purchase the Gilbert Stuart painting of George Washington known as the Lansdowne portrait. The towering image, painted from life in 1796, was an icon of the Smithsonian but was headed to the auction block. Its British owner, whose family lent the work to the museum in 1968, had decided to sell.

"This was an unusual emergency and an unusual and extraordinarily quick response," Marc Pachter, the director of the Portrait Gallery, said at the time. The Reynolds gift also enabled the Smithsonian to send the portrait on tour.

In this case, the gift of $45 million is being directed to the renovation of the Old Patent Office Building -- which houses both museums -- and future exhibitions, officials at the Smithsonian confirmed. The landmark building has been closed since 2000 for a total renovation, which includes a controversial glass canopy.

The Smithsonian announced yesterday that details of the donation would be discussed at a news conference today at the Old Patent Office Building.

Parts of the Patent Office project have been troubled, and at times raising private money for the $216 million renovation has been slow even as costs have escalated. In 2004 the Smithsonian selected acclaimed architect Norman Foster to design the glass canopy, which will cover the 28,000-square-foot courtyard between the two buildings. The Commission of Fine Arts approved the design in January. Another influential panel, the National Capital Planning Commission, approved some preliminary plans but then rejected the Foster approach in June. Last month the NCPC reversed itself and gave the go-ahead for the canopy, pending some alterations in the type of glass and lighting in the courtyard. It also ordered the restoration of the original steps along F Street NW and other historic elements.

The canopy is scheduled to be completed in 2007, a year after renovations to the rest of the 382,000-square-foot building. The Washington firm of Hartman-Cox Architects is overseeing the project.

Built between 1836 and 1867, the structure is one of the oldest buildings in Washington. Over the decades the building has served many roles; it was transferred to the Smithsonian by Congress in 1962.

The federal government has appropriated $166 million for the restoration, with the rest coming from private sources. The largest nongovernmental donation to date was $25 million, given last year by Washington philanthropists Robert and Arlene Kogod.

The Smithsonian's major donor to date has been the Kenneth E. Behring family, which gave $80 million to the National Museum of American History in 2000 and $20 million to the National Museum of Natural History. Its largess enabled the Natural History Museum to do an expansive renovation of its rotunda and mammal hall. The money at American History is also going toward renovations, and the building now also carries the Behring name.

To complete the annex for the National Air and Space Museum at Dulles International Airport, Steven F. Udvar-Hazy donated $65 million in 1999 and 2000. The Jerome and Dorothy Lemelson Foundation has given $41.5 million to the American History Museum.

In response to the Reynolds foundation's last gift, the Smithsonian decided that a gallery would be devoted to the Lansdowne painting and that the space would bear the Reynolds name. The foundation was started in 1954 by Donald Reynolds, who made his fortune in newspapers and other media businesses in Nevada, Oklahoma and Arkansas.

The $216 million renovation of the Old Patent Office Building includes architect Norman Foster's glass canopy over the courtyard.