Captopril (0.15-10 mg/kg) administration in the anesthetized dog causes immediate hypotension concomitant with an increase in tonus of the assay tissue (cat terminal ileum) superfused with circulating blood (Vane's cascade method). The increase in cat terminal ileum tonus was antagonized by a bradykinin receptor antagonist, L-349b. Treatment of the animals with indomethacin blocked or reversed the hypotensive effect of captopril without affecting the increase in tonus of the cat terminal ileum. Captopril potentiated the hypotension induced by bradykinin injected intra-arterially, and indomethacin reduced the hypotensive effect of intra-arterially injected bradykinin. Addition of captopril or enalapril to the superfusing blood maintained at 37 degrees C in an extracorporeal circuit caused a long-lasting increase in the tonus of the cat terminal ileum. The present results support the hypothesis that immediate hypotension induced by captopril involves a prostaglandin-dependent component possibly resulting from increased bradykinin levels generated in the vicinity of captopril action.