Adenylyl cyclase (AC) activity was measured in membrane preparations of platelets from control and alcoholic subjects. The sample consisted of 51 alcoholics who were categorized as type I or type II using the criteria of Gilligan et al. (Genet. Epidemiol. 4:395-414, 1987) and 54 normal controls. Alcoholic males exhibited significantly lower values than controls in basal and fluoride-stimulated platelet AC activity. When male alcoholics were segregated into type I and type II categories, the platelet AC activity did not differ between subtypes, and both subtypes had AC activity that was below control values. Western blot analysis of the quantity of Gs alpha and Gi alpha proteins in a subset of male controls and alcoholic subjects demonstrated no significant relationship between quantity of G proteins and AC activity. The results confirm lower platelet AC activity in male alcoholics, compared with controls. Given the lack of quantitative relations between Gs alpha and Gi alpha proteins and AC activity, the results support the contention that individual differences in platelet AC activity in the alcoholic subjects may reflect quantitative or qualitative differences in the AC catalytic units.