Peter Robertson Weitz, 55, a political scientist, consultant and former official of the German Marshall Plan of the United States, died of brain cancer Sept. 7 at George Washington University Hospital.

Mr. Weitz was a Washington native and grew up in Turkey and Europe, where his father was a United Nations official. He graduated from Brown University and received a master's degree in Middle Eastern studies from Harvard University. He did further graduate work at the Massachusetts Institute of Technology. In addition to English, he spoke Turkish, Italian, German and French.

In 1972, he went to work for the German Marshall Plan, an independent American foundation that was created that year by the West German government. Its purpose is to commemorate the U.S. Marshall Plan, the massive U.S. aid program that revived the economies of Western Europe after World War II. It supports public policy research, conferences, exchanges and similar projects aimed at promoting better understanding between the United States and Europe, with a particular emphasis on U.S.-German relations.

Mr. Weitz was its first program officer, and he played an important role in guiding its work. Over the years, it made grants to organizations such as the Brookings Institution, the Heritage Foundation and National Public Radio. Since the collapse of communism in the early 1990s, it has undertaken a number of programs to promote democracy and the transition to market economies in Eastern and Central Europe.

In 1997, Mr. Weitz resigned to become deputy director of the Institute for International Economics, a think tank that the German Marshall Plan established in 1981. For the past year, he had been an independent consultant on international affairs and was engaged in projects in Eastern Europe and Central Asia.

Mr. Weitz, who lived in Washington, lectured at the Foreign Service Institute and the Government Finance Corporation. He was a member of the Council on Foreign Relations, the Industrial Relations Research Association and the National Economists Club.

He was a past president of the board of trustees of Georgetown Day School.

Survivors include his wife, Judith Humphreys Weitz of Washington; two children, Jessica Weitz of Marlboro, Vt., and Thomas Weitz of New York City; his parents, Charles and Gretchen Weitz of Portland and Vinalhaven, Maine; and a sister, Barbara Weitz of Madison, Wis.