Paul F. Dougnerty, 65, a retired FBI special agent who was active in helping the mentally retarded, died Friday at Arlington Hospital after a heart attack.

He had joined the FBI in Iowa in 1942, served in San Francisco, and came to the Washington offices about 1950. He was sent on various assignments, including South America, before he retired in 1970.

Mr. Dougherty began working to aid the mentally retarded while he was still with the FBI. A son, who had suffered from Downs syndrome, had died in infancy.

In the mid-1950s, Mr. Dougherty co-founded and became president of the Arlington Association of Retarded Citizens. He was chairman of a steering committee to develop the George Mason Center in Arlington to help retarded children.

In the mid-1960s, he belonged to a group that established the Regional Residential Center for Mentally Retarded Persons in Northern Virginia, which became the Northern Virginia Training Center for the Mentally Retarded in Fairfax.

In 1970, Mr. Dougherty was named vice chairman of the Fairfax-Falls Church Community Mental Health and Mental Retardation Services Board, later serving as coordinator and then director of mental retardation.

He retired in 1976. Since then he had been active with an ecumenical group at St. John's Catholic Parish in McLean, which provides religious education for the mentally retarded. He also coordinated a special needs group for senior citizens at St. Luke's Catholic Parish in McLean.

Mr. Dougherty, who lived in McLean, was born in Sioux City, Iowa. He was a graduate of Loras College in Dubuque, Iowa, and held a law degree from Creighton University in Omaha, Neb.

He is survived by a daughter, Ann Dougherty, of Bethesda; a son, Joseph, a student at Harvard University; a sister, Mary Clare Dougherty, of California; and two brothers, the Rev. Robert E., of Iowa, and the Rev. Phillip, of Nebraska.

The family suggests that expressions of sympathy may be in the form of contributions to the Central Fairfax Services for Retarded Persons in Fairfax.