Dixon Donnelley, 66, a specialist in Latin American and economic affairs who was an assistant secretary of state for public affairs in the Johnson administration and was an assistant to former Treasury secretary Douglas Dillon, committed suicide Wednesday at his home in Bethesda.

Mr. Donnelley, a former journalist, was assistant secretary of state from 1966 to 1969. He became special assistant to Douglas Dillon in 1961, when Dillon became secretary of the Treasury. He served in the same capacity with Secretary Henry H. Fowler after Dillon's resignation in 1965. He first worked for Dillon in the late 1950s, when Dillon was under secretary of state.

After leaving the State Department in 1969, Mr. Donnelley returned for a short time to his old job at Treasury, retiring from government service later that year. He then worked as a private consultant here and overseas.

A native of New York City, Mr. Donnelley attended Columbia University. He began his career as a journalist in the 1930s with the New York Daily News. He later worked for The Washington Post and the old Washington Daily News. He joined the Foreign Service after World War II service as an intelligence officer in the Army Air Forces.

In the early 1950s, he edited a U.S.-owned news magazine, Visao, in Brazil. Earlier, he had been a press attache to three Latin American nations and worked for Nelson Rockefeller's Office of the Coordinator of Inter-American Affairs.

He was a public relations man for Sen. Estes Kefauver (D-Tenn.), when Kefauver ran for the Democratic presidential nomination in 1956 and was a consultant to President Eisenhower's Committee on Scientists and Engineers from 1956 to 1958.

Mr. Donnelley received exceptional service awards from Dillon and Fowler and from the State Department.

He was the author of a book, "Establishing and Operating a Small Newspaper."

His memberships included the Foreign Service Association, the National Press Club, the Federal City Club, the Metropolitan Club and the International Club in Washington and the Overseas Press Club of New York City.

Survivors include his wife, the former Lucia Tarquinio de Sousa, whom he married in 1943, and a daughter, Leigh D. Allen, both of Bethesda, and two grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Little Sisters of the Poor, P.O. Box 9318, Baltimore, Md., 21228.