Dr. Guido Edward Hilbert, 81, an administrator and scientist who worked for the Agriculture Department for 35 years before retiring in 1965, died of cancer Nov. 23 at a nursing home in Sun City Center, Fla. A former Bethesda resident, he had lived in Sun City Center since about 1970.

At the time he retired, Dr. Hilbert was assistant administrator of the Agricultural Research Service, where he directed foreign research and technical programs. He held that post for 11 years.

Dr. Hilbert joined Agriculture in 1930 as an associate chemist. He became head of the organic investigations section of the fertilizer and fixed nitrogen branch of the old Bureau of Chemistry and Soils.

From 1940 to 1948, he was stationed at Agriculture's Northern Regional Laboratory in Peoria, Ill. He was head of its starch and dextrose division and then director of the laboratory.

He returned to Washington as chief of the Bureau of Agriculture and Industrial Chemistry until he was named assistant administrator of the Agricultural Research Service.

Dr. Hilbert was an authority in carbohydrate chemistry and the chemistry of cereal grains. He was a member of the American Association of Cereal Chemists, the American Soybean Association and the Cosmos Club. He was a 1950 recipient of a Rockefeller Public Service Award.

He was a native of Holyoke, Mass., and a 1924 graduate of Rensselaer Polytechnic Institute in Troy, N.Y. He earned a master's degree in chemistry at Lafayette College and a doctorate in organic chemistry at Yale University.

Survivors include his wife, Martha, of Sun City Center, and a sister, Rosamumd Law of Washington, Ill.