Dr. Herman David Mast, 78, a dentist who practiced in Washington from 1925 until his retirement early this year, died Tuesday at the Washington Hospital Center. He had a heart ailment.

Dr. Mast was born in Angola, Ind. He earned his dental degree at the University of Michigan in 1924 and taught at the university clinic for a year before moving to Washington.

When he moved here, he was one of the first tenants of the old Columbia Medical Building. He remained there until 1977, when Metro construction forced its demolition.For the last 19 years, he had practiced with his son, Dr. David Mast.

The senior Dr. Mast was a member of Omicron Kappa Upsilon, an honorary dental society, and was an honorary life member of the American Dental Association and the D.C. Dental Society. He also was a member of the Washington Dental Dinner Club.

Dr. Mast was a member of St. Patrick's Episcopal Church and had served on its vestry.

In addition to Dr. David Mast of Washington, survivors include his wife, Gertrude, whom he married 55 years ago, of Washington; another son, Joseph, of Toledo, Ohio; two sisters, Florence Mast and Margaret Newman, both of Peru, Ind.; eight grandchildren and one great-grandson.

The family suggests that expressions of sympathy be in the form of contributions to the Cardiology Unit, Washington Hospital Center, 110 Irving St. NW, Washington, D.C., 20010.