Rita Regina Merthan, 50, the personal secretary to first lady Rosalynn Carter from 1977 to 1981 and a member of the board of the National Woman's Democratic Club, died Friday at Georgetown University Hospital. She was admitted Thursday after suffering a cardiac arrest.

A native of Worcester, Mass., Mrs. Merthan attended Boston University and the University of Maryland program in Frankfurt, Germany. Before joining Mrs. Carter's staff, she worked for the Central Intelligence Agency for 14 years and was an executive secretary there.

Mrs. Carter issued a statement in Plains, Ga., saying she and President Carter were "shocked and saddened by Rita Merthan's untimely death. Rita was a highly qualified professional assistant who brought loyalty, competence and sound judgment to her work in the White House. She was also a close friend and an enjoyable companion because of her personal charm and sense of humor."

In addition to serving on the board of the National Woman's Democratic Club, Mrs. Merthan was its recording secretary. She lived in Washington.

She is survived by her husband, Lawrence M. Merthan, and a daughter, Molly, both of Washington; her mother, Mary Chapowicki of Worcester; five sisters, Alice Chapowicki and Irene Paradis, both of Worcester, Lottie Dolan, of Orlando, Fla., Elizabeth Shedlick of Paris and Genevieve T. Giuffrida of Falls Church, and three brothers, Stanley, Edwin H. and Robert J. Chadwick, all of Worcester.