Laos in the period 1965-1975 provided an opportunity to study sex differences in drug and alcohol use, as influenced by ethnicity. Several psychoactive substances were locally consumed, including opium, heroin, alcohol, tobacco, betel-areca, and cannabis. Much diversity occurred among the various ethnic groups with regard to male-female use of drugs and alcohol. Trends in these use patterns suggested the existence of certain principles which govern the male-female dimension of drug use. Social changes going on in the society were reflected in choice of substance forms by younger people as compared to their elders (e.g., cigarettes vs pipes or cigars, heroin vs opium, manufactured vs village-produced alcohol). Ecological factors, which contributed to drug availability, also were powerful in determining type of drugs and patterns of use.