Leon Chatelain Jr., 77, a noted Washington architect and a former president of the American Institute of Architects, died Sunday at Doctors Hospital in Washington. He had Parkinson's disease.

Mr. Chatelain was born in Washington. He graduated from McKinley Technical High School and then studied at George Washington University. He qualified as an architect without ever getting a degree in that discipline and went on to a career that included the design of several well-known buildings in Washington and elsewhere. He also participated in numerous civic activities.

Among the buildings Mr. Chatelain designed are the headquarters of the Equitable Life Insurance Co. on Wisconsin Avenue NW, the Westmoreland Congregational Church on Westmoreland Circle, the Kiplinger Building, the headquarters of the Washington Gas Light Co., and the Suffridge Building at 1755 K St. NW.

Mr. Chatelain began his architectural practice in Washington in 1930. In 1956, he formed the partnership of Chatelain, Gauger and Nolan and in 1970 formed the firm of Chatelain, Samperton and Nolan. He retired in 1974.

He was president of the American Institute of Architects from 1956 to 1958. He was a president of the Washington Chapter of the A.I.A. and a founder and past president of the Washington Building Congress. He also was chairman of the National Commission on Architectural Barriers to the Handicapped and a member of the President's Committee for Employment of the Handicapped.

Mr. Chatelain was a past president of the Washigton Board of Trade, the Kiwanis Club of Washington, the Metropolitan Police Boy's Club, the D.C. Crippled Children and the National Easter Seal Society. He was a founder of Heroes Inc., and a trustee of George Washington University.

He received many awards, including a citation for his service on the President's Committee for Employment of the Handicapped in 1962. He was a director of several businesses.

He was a Mason and a member of the Capitol Hill Circus Saints and Sinners, Congressional Country and Cosmos clubs. He was a Mason and a member of the Almas Shine.

Mr. Chatelain was a steward of the Metropolitan Memorial Methodist Church.

Survivors include his wife, Mary Wysong Chatelain, of the home in Washington; a daughter, Jo Ann Wood, of New York City; two sons, Edward R., of North Carolian, and Leon III, of Waterford, Va.; a sister, Alice Hutchins, of Madison, Wis., and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the D.C. Society for Crippled Children, to Heroes Inc., or to the Kiwanis Foundation of Washington, D.C. CAPTION: Picture, LEON CHATELAIN JR.