The renin-angiotensin system plays an important role in maintaining blood pressure in various pathologic and physiologic states. To investigate the effects of the sodium nitroprusside (SNP)-induced hypotension on plasma renin activity and serum aldosterone level, SNP 2.0 mg/kg/min was infused to seven unanesthetized rabbits. The mean arterial pressure and heart rate were measured three times; at the start of, 15 and 30 minutes after SNP inusion and 30 minutes after the discontinuation of SNP infusion. The measurement of the plasma renin activity and the serum aldosterone levels during SNP infusion were done by means of radioimmunoassay. The results were as follows: 1) The mean arterial pressure was 117.6+/-6.9 mmHg at zero time ans decreased significantly to 84.0+/-19.6mmHg and 72.9+/-21.6mmHg at 15 and 30 minutes after SNP infusion, respectiely(P<0.01). 2) The heart rate was 124.8+/-9.3 beats/min at zero time and increased significantly to 139.7+/-6.4 beats/minute and 155.6+/-7.9 beats at 15 and 30 minutes after SNP infusion, respectively(P<0.05). 3) The plasma renin activity was 2.31+/-0.53 ng/ml/hr at zero time and increased significantly to 5.17+/-1.39 ng/ml/hr and 4.97+/-1.52 ng/ml/hr in 15 and 30 minutes after SNP infusion, respectively. 4) The serum aldosterone level was 28.8+/-13.5ng/dl at zero time and increased to 42.3+/-14.6ng/dl and 39.5+/-13.9ng/dl at 15 and 30 minutes after SNP infusion and it continued to increase up to 30 minutes after discontinuation of SNP infusion. In conclusion, it was postulated that the plasma renin activity and serum aldosterone level were closely related to the change of the mean arterial pressure and heart rate during SNP-induced hypotension.