1. Vasodilator responses to bradykinin (BK) in the rat heart are reported to be independent of NO and cyclo-oxygenase/lipoxygenase products of arachidonic acid (AA). 2. We verified that inhibition of NO synthase with L-nitroarginine (50 microM) and cyclo-oxygenase with indomethacin (2.8 microM) were without effect on vasodilator responses to BK (10-1000 ng) in the Langendorff rat heart preparation. 3. L-Nitroarginine elevated perfusion pressure, signifying a crucial role of NO in the maintenance of basal vasculature tone. 4. In hearts treated with L-nitroarginine to eliminate NO and elevate perfusion pressure, vasodilator responses were reduced by inhibitors of cytochrome P450 (P450), clotrimazole (1 microM) and 7-ethoxyresorufin (1 microM). 17-Octadecynoic acid (17-ODYA 2 microM), a mechanism based inhibitor of P450-dependent metabolism of fatty acids, also reduced vasodilator responses to BK. 5. These results confirm that NO and prostaglandins do not mediate vasodilator responses to BK in the rat heart but suggest a major role for a P450-dependent mechanism via AA metabolism.