Hemodynamic and blood-gas variables were studied before and after pentolinium tartrate administration in six patients anesthetized with nitrous oxide–halothane and maintained at PaCO235–40 torr. Measurements were made prior to induction of anesthesia; before and 10, 20, and 60 minutes after administration of pentolinium (0.3 mg/kg); 15 minutes after return of arterial blood pressure to control values. Mean arterial blood pressure (MAP) was significantly decreased at 20 (P< 0.02) and 60 (P< 0.001) minutes, in association with significant decreases in systemic vascular resistance (SVR) (P< 0.05 andP< 0.005). At 60 minutes MAP was significantly lower than that at 10 minutes (P< 0.01). Cardiac output (CO) was increased (P< 0.05) after 10 minutes secondary to a significant increase in heart rate. Neither variable changed significantly thereafter. CO and HR were significantly lower (P< 0.01) 60 minutes after pentolinium than at 10 minutes; both returned to 10-minute values after intravenous administration of atropine. Changes in stroke volume (SV) and mean right atrial pressure (MRAP) were not significant. Whole-body O2uptake (&OV0312;O2) was not significantly altered by pentolinium. However, a substantial diminution of myocardial O2consumption (M&OV0312;O2) was deduced from a significant decrease in the heart rate–arterial systolic pressure product (HR × ASP). Fifteen minutes after return of MAP to control levels, SVR was 11.5 per cent lower, while CO was still significantly higher (P< 0.02) than control values. Following ganglionic blockade with pentolinium during halothane–N2O anesthesia, HR is a valuable index of changes in CO, while the HR × ASP index may be utilized to evaluate changes in M&OV0312;O2. Assessment of myocardial performance during controlled hypotension is possible by the use of routinely available measurements.