Rep. John M. Slack Jr., 64, a West Virginia Democrat who had served in the House of Representatives since 1959, died Monday at Mt. Vernon Hospital in Alexandria after a heart attack.

He was a member of the House Appropriations Committee and was chairman of its subcommittee on State, Justice, Commerce and the Judiciary. He also was a member of the House Ethics Committee.

Mr. Slack was a native of West Virginia and attended the Virginia Military Institute. He served in the Army during World War II.

He was in the real estate business in Kanawha County and was county assessor for six years before winning election to Congress in 1958.

During his years in Congress he was regarded by observers as more conservative than most of his Democratic colleagues. He favored some legislation easing regulations on some types of strip mining.

Mr. Slack was a critic of the Federal Trade Commission's handling of antitrust investigations in the automobile and oil industries as well as of FTC probes on the funeral industry, health spas and apparel labeling.

Following his death, Senate Majority Leader Robert C. Byrd (D-W.Va.) said, "Our state has lost a dedicated congressman and we all share deeply in the loss. He was a good and conscientious representative."

Survivors include his wife, Frances, of Alexandria, where he lived; a son, John, of Charlestown, W.Va., and two grandchildren.