Between 1977 and 1989, there were marked reductions in the prevalence of 10 of the 17 substances included in the surveys. Four additional substances showed a significant downward trend beginning in the early 1980's. Findings in Ontario, which are presented in greater detail in this article, also indicate a decrease in multiple drug use among adolescents. The authors suggest that the overall decline in drug use is attributable not only to a reduction in demand, but also to greater global health-related behaviors and health consciousness. Some of the future trends in Canadian adolescent drug use outlined by the authors include a lower rate of illicit substance use than that estimated for comparable American age groups, enduring declines in illicit use among the current cohort of adolescents, a stabilization in the numbers of adolescents trying crack or cocaine, and the appearance of new drugs and new methods of drug administration in North America. 3 tables, 3 figures, and 20 references (Author abstract modified)