Cyclic AMP levels and activities of adenylate cyclase and phosphodiesterase in the striatum were measured in 4, 12, 24 and 30 month-old rats. No age-related changes were observed either in cyclic AMP levels or basal adenylate cyclase activity. However, there was an age-related decline in dopamine sensitivity of adenylate cyclase without affecting NaF-induced stimulation. There was also an age-dependent decrease in phosphodiesterase activity at low substrate concentrations (3 X 10(-7) and 3 X 10(-6) M) but not at high substrate concentrations (3 X 10(-5) and 3 X 10(-4) M). These results suggest that the functions of dopaminergic neurons may have been altered by aging.