The influence of debrisoquine on pressor actions of norepinephrine (NE) and tyramine (TR) was investigated in rabbits. Debrisquine(D), in the doses of 1.0, 3.0 and 6.0.mu.g/kg, i.v. potentiated significantly the pressor actions of NE and TR, except the action of TR in the dose of 1.0mg/kg of debrisoquine. NE response potentiated by debrisoquine was not affected by tranylcypromine, a MAO inhabiter, or desipramine, a NE uptake blocking agent, but augmented by reserpine, a NE depleting agent, or bethanidine, a sympathetic neuronal blocking agent. NE response potentiated by tranylcypromine or desipramine was augmented by debrisoquine, while NE response potentiated by reserine or bethanidine was not affected by debrisoquine. TR response potentiated by debrisoquine was weakened by tranylcypromine, desipramine or reserpine, and not affected by bethanidine. TR response in rabbit pretreated with tranylcypromine, desipramine or reserpine was augmented by debrisoquine, but in rabbit pretreated with bethanidine was not affected by debrisoquine.