The purpose of this study was to evaluate the efficacy of adenosine triphosephate (ATP) in comparison to sodium nitroprusside (SNP) in reducing left ventricular afterload in 20 patients, ASA physical status I, during ethrane-N2O anesthesia. Hemodynamic effects of intravenous ATP (30~250 ug/kg/min) were compared with those of SNP (0.3~2.5 ug/kg/min) in group 1 (n=10). In group 2 (n=10), hemodynamic and intrapulmonary shunt effects of dobutamine (1 ug/kg/min) alone and in combination with ATP or SNP, required to maintain mean arterial pressure around 70 mmHg, were compared. The results were as follows. 1) Both ATP and SNP reduced arterial pressure rapidly resulting from a marked decrease in systemic vascular resistance in a dose-related manner. 2) Cardiac index increased from 3.31±0.201/min/m2 to 4.04±0.281/min/m2 (p<0.01) following dobutamine alone, and increased further to 5.71±0.38 1/min/m2 (p<0.001) and decreased to 3.77±0.28 1/min/m (NS) in combination with ATP and SNP, respectively. 3) At equivalent decrease in mean arterial pressure, ATP increased heart rate significantly less than SNP. 4) Hypotensive response was more stable during ATP infusion than during SNP infusion. 5) Arterial oxygen tension was significantly higher during dobutamine/ATP infusion than during dobutamine/SNP infusion (268±6 vs 256±9 mmHg, p<0.05). 6) Intrapulmonary shunt fraction increased from 4.49±0.65% to 5.51±0.71% (p<0.05) following dobutamine alone, and increased further to 9.92±1.13 (p<0.001) and to 7.21±0.77% (p<0.01) in combination with ATP and SNP, respectively. These results suggest, although ATP increases intrapulmonary shunt fraction more than does SNP, ATP has significant advantage over SNP, either alone or in combination with dobutamine, in improving cardiac performance in patients with low output states due to high peripheral vascular resistance.