Hypotensive actions of single and repeated administrations of nifedipine were evaluated in comparison with hydralazine in normotensive, spontaneously hypertensive and renal hypertensive rats. Substantial and prolonged falls in blood pressure were observed following oral dosing with 3 mg/kg of nifedipine in renal hypertensive rats, with 10 mg/kg of nifedipine in normotensive and spontaneously hypertensive rats and with 10 mg/dg of hydralazine in all three groups of animals. The hypotensive effect of nifedipine was greater in both forms of hypertensive rats than the normotensive rats, while there was no difference in the hypotensive effect of hydralazine in normotensive and hypertensive animals. In spontaneously hypertensive rats tolerance to nifedipine and hydralazine was not observed with oral administrations for 4 weeks at a daily dose of 10 mg/kg. An increase in heart rate always accompanied the hypotension induced by both drugs, but this tachycardia was seen to a lesser extent with nifedipine than with hydralazine. The mechanism of the selective action of nifedipine in hypertensive rats in contrast to normotensive rats is discussed.