Pulmonary and systemic hemodynamic responses to administration of pentolinium (0.3 mg/kg) were studied in eight patients undergoing total hip replacement during N2O-halothane anesthesia with Paco2maintained at 35–40 mm Hg. Measurements were made prior to induction of anesthesia; 20 minutes after induction; 45 minutes after induction but prior to pentolinium administration; 10, 20, and 60 minutes after administration of pentolinium; 15 minutes after AP recovered to within 10 per cent of baseline values. The reduction in AP produced by pentolinium was associated with significant decreases in both SVR and LVFP (estimated by measuring PA0via a Swan-Ganz catheter). The profound and significant reductions in calculated PVR were more pronounced than those in SVR. Since PVR decreased in the face of reductions in both pulmonary arterial and left atrial pressures despite a significant increase (at 10 minutes) or no change (at 20 and 60 minutes) in pulmonary blood How, the results suggest an active decrease in pulmonary vascular tone, presumably due to autonomic inhibition by pentolinium. Both LVSWI and RVSWI decreased significantly following injection of pentolinium. HR × SAP, an indirect index of MVo2decreased significantly following an initial increase at 10 minutes. Presumably these effects of pentolinium were conspicuous because N2O-halothane anesthesia altered the baseline of pulmonary vascular tone.