Walter G. M. Fields Sr., 78, who had practiced law in Washington and Montgomery County for 49 years and was active in youth programs, died of congestive heart failure Thursday at Shady Grove Adventist Hospital. He lived in Rockville.

He had maintained a general law practice in Rockville until his retirement last December. He had been an active member for many years of the Montgomery County Bar Association and has established its Judicare Committee, of which he was chairman, to offer legal and counseling services to troubled youth at reduced fees. The committee was expanded to include the Legal Aid Services of Montgomery County.

Mr. Fields was born in Washington. He received his law degree in 1925 from National University and was admitted to the Florida Bar. Later he was admitted to the D.C. and Maryland bars. In 1941, he was named a member of the National Panel of Arbitrators. In 1946, he was honored by President Truman for his work with the Selective Service Adminstration.

Although he was not of the Catholic faith, Mr. Fields had been awarded the Pro Deo Et Juventute Medal by the Archdiocese of Washington for his work with youth through the Catholic Youth Organization. He was also named Coach of the Year by the Archdiocese for his work with the City League of Softball, which saw the Rockville softball program for girls greatly expanded.

Survivors include his wife, Marguerite B., of Rockville; two sons, Walter G. M. Jr., of Greenbelt, and Robert S., of Manassas; two daughters, Mary F. Boyd of Corpus Christi, Tex., and Ann McNally of Gaithersburg; a brother, Robert, of Baltimore, and three granchildren.

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