An influence of bethanidine (B) onpressor effects of norepinephrine (NE) and tyramine (TR) was investigated in the whole rabbits. B, in a dose 1.0, 3.0 and 5.0 mg/kg, i.v., potentiated significantly the pressor effects of NE and TR. Reserpine and desipramine did not increase the NE effect that had been potentiated by B.B also made little modification of NE effect that had been potentiated by reserpine and desipramine. B increased the NE effect that had been potentiated by tranycypromine and guanethidine. The NE pressor effect potentiated by B was decreased by tranylcypromine, but not influenced by guanethidine. The TR pressor effect potentiated by B was not altered by reserpine and guanethidine, but decreased by desipramine and tranylcypromine. B increased the TR pressor effect that had been potentiated by guanethidine. B, when given after administration of reserpine, tranylcypromine or desipramine, exerted little influence on the TR effect. The mechanism of potentiation of NE and TR pressor effects by B seems to be similar to guanethidine, and the potency of B on the influence of NE and TR effects seems to be greater than guanethidine.