Adenosine triphosphate (ATP) has been reported to be a hypotensive agent similar in effect to sodium nitroprusside (SNP). The purpose of this study was to examine and compare the effects of both SNP and ATP on general coronary hemodynamics, myocardial O2consumption, and circulating catecholamines. Twelve dogs were anesthetized with 1.0% halothane and given either SNP or ATP by controlled infusion to reduce their systemic blood pressure by 50% for a 2-h period followed by a (blood pressure) recovery period. The ATP-induced hypotension was rapid, easily controlled, not accompanied by tachyphylaxis over the 120 min studied, and resulted in an increase in coronary sinus blood flow (CSBF), which plateaued at 260% above control. The increase in CSBF was almost immediate and remained at this elevated level for the duration of the induced hypotension. During the ATP-induced hypotension, there was no change in heart rate or circulating catecholamines. A 60% reduction in myocardial O2uptake was observed, presumably from the cardiac unloading. In contrast, SNP-induced hypotension required a marked increase in dose over time, did not significantly increase CSBF, did increase heart rate, and resulted in large increases in circulating plasma catecholamines. Neither agent affected cardiac output. ATP-induced hypotension resulted in no change in cardiac lactic acid uptake, while SNP caused lactic acid production, indicating possible cardiac ischemia or cyanide toxicity.