Research on adolescent substance use has focused on prevalence and incidence; however, contextual factors have been largely ignored. A survey of 155 adolescents from a Minneapolis suburb was conducted to assess contextual factors affecting adolescent substance use. Subjects reported their use of alcohol, cigarettes, and marihuana with respect to where, when, and with whom each substance was used. Age was correlated with alcohol use at parties, in cars, at a friends' home, and in streets or parks. Age and alcohol use were associated with schoolmates, best friends, and nonschool friends, as well as with weekends and holidays or special occasions. Age and smoking were correlated with use at school, parties, or in cars. Cigarette use was associated with school friends, during school, and on holidays. Age and marihuana use were correlated with use at parties, in cars, at school, at friends' homes, and in streets or parks, as well as with school friends, nonschool friends, siblings, and best friends. Results showed strong relationship between age, substance use, and contextual factors. Among 12-13 year olds, alcohol was most widely used at home, while 16-17 year olds reported the most use at parties. Marihuana and age were most closely associated with use on weekends, holidays, special occasions, and during school. The findings suggest that problems of substance use and abuse should be interpreted within a larger framework. (NRB)