Circu-latory effects of nitrous oxide-d-tubocurarine anesthesia with hyper-ventilation were studied in healthy, young men. Cardiac output, mean arterial blood pressure, and splanchnic blood flow were measured. During anesthesia and hyperventilation with normal PCO2 (CO2 added to the inspired gases), splanchnic vascular resistance was elevated significantly and the blood flow reduced, but O consumption was unaltered from control values. Cardiac output and total peripheral resistance were unchanged. When PCO2 was not maintained at the normal level, total peripheral resistance and splanchnic vascular resistance were reduced and the respective blood flows augmented compared with the normocarbic period. The demand of the splanchnic viscera for O was increased out of proportion to the increase in flow. The nitrous oxide-curarehyperventilation technique may be a poor choice for patients with marginal levels of splanchnic blood flow.