A repeated measures design was used to assess the effects of a methadone maintenance program in several California locations. Subjects were 720 Chicano and Anglo men and women participating in rehabilitation programs for heroin addicts. The subjects were interviewed between 1978-1981, and the average follow-up time after initial treatment was 4-6 years. The follow-up rate was 90 percent of those alive at the time of the interview. This repeated measures design compared individuals' behaviors for a relatively long pretreatment period, in order to ensure a more stable baseline; for all periods during the treatment; and for all periods when not in treatment after the entry into initial treatment. The amount of time spent in treatment was also examined. The interview, adapted from Nurco's schedule, assessed behavior, legal problems, drug use, employment, etc., during the different time intervals. Results indicated that the time factor was significant for all outcome measures. Significant behavioral improvement was noted during times of treatment. Racial differences and sex differences were also noted. (GDC)