Iprindole is a clinically effective antidepressant similar to other tricyclic drugs in structure. In animal studies it does not appear to inhibit the levarterenol membrane pump. Investigation was made of iprindole's effect on the peripheral adrenergic levarterenol pump in man utilizing blood pressure response to tyramine and levarterenol. Other clinically effective tricyclic drugs exert, by blocking its uptake, a potent blocking effect on the blood pressure response to tyramine and, in a similar manner, augment the pressor response to infused levarterenol. In contrast, iprindole showed no significant effect on tyramine and levarterenol blood pressure response. Other tricyclics also reduce platelet serotonin content, probably by inhibiting membrane uptake of this amine. Iprindole failed to alter platelet serotonin content. These findings call into question that aspect of the biogenic amine theory of depression which states that the tricyclic drugs exert their antidepressant action by inhibiting the membrane pump.