To investigate the nature of the bradykinin-induced potentiation of electrically driven muscle twitches in the isolated vas deferens, bradykinin, noradrenaline and adenosine 5'-triphosphate (ATP) concentration-response curves were made with control, reserpinized and chemically sympathectomized rats. Bradykinin potentiated the ATP- but not the noradrenaline-induced contractions in the epididymal and prostatic segments of the ductus. The epididymal segment of the ductus did not respond to transmural electrical stimulation following reserpine treatment. Bradykinin potentiated the muscular contractions caused by exogenous ATP but not by noradrenaline. In contrast, the transmurally evoked twitches of the prostatic portion of the ductus remained almost unaltered; bradykinin increased the motor effect of ATP without modifying the potency of noradrenaline. All neuronally induced contractile activity was absent in sympathectomized rats; bradykinin potentiated the contractile effect of ATP without altering the noradrenaline-induced contractions. These results suggest that bradykinin potentiates the ATP-evoked contractions by acting postjunctionally.