Marguerite U. Arendes, 66, a former employee of the National Security Agency who was active in church groups, died at her home in Silver Spring Wednesday of cardiac arrest.

Mrs. arendes, who was born in St. Mary's County, Md., graduated from Eastern High School in Washington. She had lived in Silver Spring in recent years.

For four years before her marriage in 1934 to Joseph R. Arendes, she was employed at the photography firm of Harris and Ewing. In the 1950s, she was a secretary in the health insurance claims department at Providence Hospital and later worked at the National Security Agency. She resigned from the government in 1959.

Mrs. Arendes was a member of Ladies Auxiliary No. 50 of the Knights of St. John. She also was a member of St. Camilus Catholic Church in Silver Spring and of its Sodality. She was a volunteer at St. Peter's Catholic Church and St. Francis Xavier Catholic Church in Washington.

In addition to her husband, of Silver Spring, survivors include a daughter, Ann Marie Anderson of Silver Spring; two sons, Joseph M. of Owings, Md., and Vincent F. of Rockville; a brother, John R. Hamilton of Waldorf, Md., and seven grandchildren.

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