These experiments were designed to study the effects of inhibition of endogenous bradykinin and/or prostaglandins on systemic and regional hemodynamics in conscious normotensive rats, using the radioactive microsphere technique. Administration of either a bradykinin antagonist (50 micrograms/min for 5 min) or indomethacin (10 mg/kg) alone, decreased the cardiac output by an average of 17.58 and 25.94%, respectively (P less than 0.05), without significant change in total peripheral resistance. Regional blood flow decreased and local vascular resistance increased in most organs. Coronary and renal blood flow decreased by an average of 16.45 and 17.26% with bradykinin antagonist and 23.85 and 19.80% with indomethacin, respectively (P less than 0.05). Indomethacin but not bradykinin antagonist also decreased cerebral blood flow by an average of 47.57% (P less than 0.01). Infusion of the bradykinin antagonist following prior prostaglandin inhibition with indomethacin did not induce further changes in either systemic or regional hemodynamics, except in the liver where a significant additional increment in vascular resistance was observed. Our data suggest that most organs have similar patterns of regional vascular sensitivity to bradykinin and prostaglandins (including the heart, kidney, testis, stomach, skin and adrenal). The major differences were in the cerebral vasculature, which was much more sensitive to prostaglandins, and in the liver, spleen and small intestine, which were more sensitive to bradykinin. We conclude that both hormonal systems participate to a varying extent in the maintenance of resting vascular tone in most organs; however, in some cases, their effects may be additive and in others they may work in opposite directions.