Incidence of marijuana use began increasing in the 1960's and reached a peak in 1973, after which a continuing decline occurred. Cocaine use incidence began its increase in the late 1960's, reached a peak in 1982, and declined subsequently. Trends in hallucinogen incidence appear similar to those for marijuana, but decreases since the 1972!1974 peak do not seem as marked as those for marijuana. Estimates of incidence of heroin use are unstable due to the small number of users, but data show a peak period for initiation during 1971!1977. 4 tables and 8 references