Dr. Mari Nyswander, 67, a psychiatrist who helped develop methadone treatment for heroin addiction and who wrote a book that suggested treating drug addiction as a medical problem, died of cancer April 20 at her home in New York City.

In partnership with Dr. Vincent P. Dole, whom she married in 1965, Dr. Nyswander developed the methadone treatment at Rockefeller University in New York. She did much of her research in a storefront clinic in Harlem.

A synthetic narcotic, methadone relieves the addict's need for heroin without causing the highs and lows of heroin use. About 150,000 heroin addicts have used methadone, with varying degrees of success.

At the time of her death, Dr. Nyswander was a senior research associate at Rockefeller University and an assistant clinical professor at Columbia University and at New York Medical College.

She earned awards from the American Public Health Association, the National Drug Abuse Conference and the New York Urban League. She also served on a presidential commission on psychoactive drugs.

Dr. Nyswander was associated with Beth Israel Medical Center, where she and her husband instituted trials of methadone in 1964. She was affiliated for more than a decade with St. Clare's Hospital, and at various times with New York Hospital, the Albert Einstein College of Medicine, Harlem Hospital and Bronx Municipal Hospital.

Dr. Nyswander was born in Reno, Nev. She was a 1941 graduate of Sarah Lawrence College and earned her medical degree at Cornell University in 1944. Her introduction to the problems of drug addiction came during World War II when she was a lieutenant with the U.S. Public Health Service in Lexington, Ky.

From 1947 to 1950, she trained in psychiatry and psychoanalysis at New York Medical College, and completed her residency at Bellevue Hospital. In 1956, she published "The Drug Addict as a Patient," which argued that addiction is a medical problem.

In addition to her husband, of New York City, survivors include her mother, Dorothy Nyswander of Berkeley, Calif.; a half sister, Ruth Del Duca of Menlo Park, Calif., and three stepchildren.