Benton J. Stong, 75, administrative assistant to Sen. John Melcher (D-Mont.) and a veteran of 25 years as a staff member on Capitol Hill, died of cardiac arrest Saturday at Sibley Memorial Hospital.

He became administrative assistant to the then-Rep. Melcher in 1969, and continued as administrative assistant following Melcher's election to the Senate in 1976.

Before joining Melcher's office, Mr. Stong had spent nine years on the staff of Sen. George McGovern (D-S.D.) and had been McGovern's legislative assistant.

Mr. Stong came to Capitol Hill in 1955 to join the staff of Senate Interior and Insular Affairs Committee, where he worked until joining McGovern in 1963.

Mr. Stong was recognized as an authority on farm policy and was credited with drafting the Voluntary Wheat Certificate Plan of 1964, successfully sponsored by Sen. McGovern. Mr. Stong also helped draft the first proposals for the Wilderness Act, which were introduced by former Sen. James E. Murray (D-Mont.) in the 1950s.

Mr. Stong's awards included the American Motors Conservation Award in 1964 and the George W. Norris Award of the Western States Water and Power Consumers Conference in 1966. He was a life member of the Soil Conservation Society of America.

He was a native of Iowa where he graduated from Drake University with a degree in journalism. He was a reporter with The Des Moines Register and was editor of The Knoxville (Tenn.) News Sentinel before coming to Washington as a reporter with the Scripps-Howard newspapers in 1942.

Mr. Stong was associated with the National Farmers Union for many years and was an editorial consultant for that organization's publication, the "Reporter," during the 1970s.

He was a gardener and had won local awards for dahlias he grew.

Mr. Stong's surviors include his wife, Laverne of Falls Church; a daughter, Norma Lyon of Toledo; three sons, Benton J. III, of Summer, Iowa; Michael, of Vienna, and Joe, of San Francisco; 12 grandchildren, and one great-grandchild.

The family suggests that expressions of sympathy be in the form of contributions to the American Heart Association.