The analyses are based on a followup in 1980-81 of a sample representative of adolescents formerly enrolled in grades 10 and 11 in public secondary schools in New York State in the fall of 1971. The data was obtained through personal household interviews based on a structured schedule and two charts designed to reconstruct monthly the respondents' drug and life histories. Information was collected on the histories of use of 12 drugs: two legal, four illegal, and six drugs that should be used only under medical prescription but that are also used on one's own. In this cohort, the period of major risk for initiation to cigarettes, alcohol, and marijuana is completed for the most part by age 20, and to illicit drugs other than cocaine by age 21. Those who have not experimented with any of these substances by that age are unlikely to do so thereafter. Initiation into prescribed psychoactive drug occurs at a later age than for the licit and illicit drugs and continues through the age period covered by the survey. A potential maturational trend in marijuana use in this cohort is apparent, with a decline beginning approximately at age 22.5 for most usage patterns. The periods of highest marijuana and alcohol usage declined beginning at ages 20-21 and contrast sharply with cigarettes, which exhibit climbing rates of highest use through the end of the surveillance period (age 25). Overall patterns are similar for men and women, with men initiating all drugs at higher rates than women, except for prescribed psychoactives. 1 table, 3 figures, and 11 references