The United States armed forces adopted "zero tolerance" policies concerning illicit drug use in 1980, and later developed policies to discourage tobacco and alcohol abuse. This paper examines drug use among young active-duty recruits both before and after enlistment, compared with non-military age-mates. It also documents historical shifts in such drug use across two decades. Analyses employed longitudinal panel data from 20 nationally representative samples of high school seniors (cohorts of 1976-1995) each surveyed just before graduation and again within two years. Separate analyses for men (n=12,082) and women (n=15,345) contrasted those who entered military service, college, and civilian employment. Results show that illicit drug use declined more among young military recruits than among their civilian counterparts. Analyses of male recruits at multiple time periods showed declines in the prevalence of marijuana use and cocaine use after the initiation of routine military drug testing. Lower proportions of smokers of half a pack or more cigarettes per day elected to enter service after initiation of tobacco bans during basic training. An appendix discusses racial/ethic differences. Tables contrast substance use between high school seniors, college students, civilians, and those in military service. (Contains 11 tables, 3 figures, and 29 references.) (Author/MKA)