Sodium nitroprusside is used to induce hypotension to decrease bleeding in operation site, Sodium nitroprusside decrease vascular resistance by directly relax arteriolar and venous smooth muscle, to a lesser extent. Hypotension stimulate baroreceptors and increase sympatho-adrenal activity. The function of sympatho-adrenal mdullary system may be monitored by the changes of plasma cancentrations of epinephrine and norepinephrine. To study the relationship between induced hypotension and the catecholamine release, the plasma epinephrine and norepinephrine concentration were measured before, during and after infusion of aodium nitropruaside to rabbits The results were as follows: 1) The mean arterial blood pressure decreased significantly after SNP infusion (5.0 ug/kg/minute) from the control value of 98.1±11.9 mmHg to 66.3±16.2, 57.9±17.9 mmHg in 10 and 20 minutes after SNP infusion (P<0.01) and 78.8±12.5 mmHg after discontinuation of SNP infusion. 2) The heart rate increased significantly after SNP infusion from the control value of 149.8±12.8 beat/minute to 166.5±10.8, 190.4±17.6 beats/minute in 10 and 20 minutes after SNP infusion (P<0.05, P<0.01) and the heart rate increased after discontinuation of SNP infusion. 3) The plasma epinephrine concentration increased significantly after SNP infusion from the control value of 181.2±91.9 pg/ml to 547.5±163.2, 837.5±253.6 pg/ml in 10 and 20 minutes after SNP infusion and continued to increase of epinephrine after discontinuation of SNP infusion. 4) The plasma norepinephrine concentration increased significantly after SNP infusion from the control value of 566.2±92.6 pg/ml to 1131.3±424.7, 1432.5±479.2 pg/ml 10 and 20 minutes after SNP infusion and continued to increase after discontinuation of SNP infusion. There was a highly significant correlation between decrease in mean arterial pressure and increase in plasma epinephrine and norepinephrine concentrations.