The effect of a single oral dose of tripelennamine, 100 mg., methyldopa, 500 mg., d‐amphetamine, 10 mg., amitriptyline, 50 mg., or prochlorperazine, 25 mg., on the blood pressures of hypertensive patients treated chronically with guanethidine was measured half‐hourly for 8 hours. D‐amphetamine antagonized the antihypertensive effect of guanethidine in the lying systolic blood pressure, while the orthostatic hypotension persisted. The effect of d‐amphetamine seems to be due to the direct stimulation of vasoconstrictor receptors. Tripelennamine, amitriptyline and prochlorperazine had no demonstrable short‐term effect on the guanethidine action. Methyldopa potentiated guanethidine in the standing diastolic blood pressure.