Bradford Morse, the highest-ranking American on the United Nations secretariat, plans to retire as undersecretary general and administrator of the U.N. Development Fund within a few months, his office announced yesterday.

Morse, 64, a former Republican member of Congress from Massachusetts, has been with the secretariat since 1972 when he succeeded the late Ralph Bunche as undersecretary for General Assembly and political affairs. In 1975, he became head of the fund overseeing all U.N. development activities and technical assistance for underdeveloped nations.

Since last December, Morse also has been in charge of U.N. efforts to ease effects of drought-caused famine in Africa.

Since the inception of the United Nations in 1945, the United States, as a permanent member of the Security Council and largest contributor of U.N. funds, has implicitly controlled one undersecretary position. U.N. sources said yesterday that they expect this to continue but that it is too early to say who might succeed Morse.

A spokesman for Morse, who is traveling in Europe, said he is retiring "for personal reasons" and plans to leave at a time agreeable to Secretary General Javier Perez de Cuellar.

The spokesman said Morse will continue various other activities including the chairmanship of the Salzburg Seminars, an Austria-based forum for conferences on world issues.