Ruth I. Cotton, 85, the wife of former senator Norris H. Cotton (R-N.H.), died Thursday at the Alice Peck Day Extended Care Facility in ebanon, N.H., which she had entered four years ago because of poor health.

Her husband had cited her health as one of his reasons for resigning from the Senate in 1974 after 20 years there. He served again briefly by appointment in 1975 pending result of a special New Hampshire election to fill a Senate seat.

Mrs. Cotton was born in Lynnville, Tenn. She graduated from McLean College in Kentucky. She came to Washington in the early 1920s as a secretary to Sen. Kenneth D. Kellar (D-Tenn.).

She met her husband, then a secretary to Sen. George H. Moses (R.N.H.), on the Hill, and they were married in 1927. They lived in Concord, N.H., where he was a lawyer, until moving to Lebanon in 1933. They had maintained a home there since then.

They came back to Washington in 1947, when Cotton was elected to the House of Representatives. He resigned in 1954 and ran successfully for the Senate.

Mrs. Cutton was a member of the Daughters of the Confederacy.

Her husband is her only immediate survivor.