Robert G. Dixon, 60, a former assistant attorney general of the United States and a leading authority on constitutional and administrative law, died of a heart attack Monday at Johns Hopkins Hospital in Baltimore. lHe was stricken while undergoing surgery for a circulatory disorder in one of his legs.

Mr. Dixon was assistant attorney general, office of legal counsel, from 1973 to 1974. He taught political science at the University of Maryland from 1949 to 1956 and was a professor of law at George Washington University from 1956 to 1975. He was Daniel Noyes Kirby Professor of Law at Washington Univeristy in St. Louis, at the time of his death. He taught at the Univeristy of Virginia Law School in Charlottesville as a visiting professor this spring.

Mr. Dixon was brought into the Justice Department by former attorney general Elliott L. Richardson. Among the things he did there was conduct a study of the law of impeachment in view of the possiblity that President Nixon would be tried by the Senate in connection with the Watergate scandal. nHe concluded that if a president tried to withhold evidence in such a trial "a constitutional confrontation of the highest magnitude would ensue."

But much of Mr. Dixon's career was devoted to the law of reapportioning legislative districts. His book, "Democratic Representation: Reapportionment in Law and Politics," won the Woodrow Wilson Foundation Book Award in 1968 and is considered a leading text on that subject.

Mr. Dixon also was an expert in administrative law and a member of the Administrative Conference of the United States.

A native of Canajoharie, N.Y., Mr. Dixon earned bachelor's and doctoral degrees in political science at Syracuse University.He earned his law degree from George Washington University in 1956 and joined the faculty there in the same year. He lived in Rockville before moving to St. Louis.

In the course of his career, Mr. Dixon received fellowships from the Rockfeller and Ford foundations and the National Endowment for the Humanities. He received the George Washington Univeristy alumni Achievement Award in 1978.

Survivors include his wife, Claire, of St. Louis, and three daughters, Mrs. James Ryan of Charlottesville, Mrs. Walter Teagle of New York City, and Laurie Dixon of St. Louis.