The time course and magnitude of change in mean arterial pressure (MAP) and heart rate (HR) following a single rapid intravenous injection of 1µg/kg and 2µg/kg of sodium nitro prusside(SNP) were determined in 10 anesthetized patients. The results were as follows: 1) MAP began to decrease 15 seconds after both 1µg/kg and 2µg/kg of SNP and was maximal after 90 seconds with return toward control apparent by 3 minutes and complete after 5 to 6 minutes(Fig. 1). 2) The magnitude of decrease in MAP after SNP was dose related whereas onset and duration of blood pressure changes were not influenced by the SNP dose. 3) HR did not change significantly after either dose of SNP. SNP is a potent, rapid acting antihypertensive drug which produces arteriolar vasodilation and prevention of hypertension and is particularly important in patients with decreased myocardial reserve or intracranial pathology.