The hemodynamic effects of a verapamil infusion were investigated in six dogs given fentanyl-nitrous oxide anesthesia. Verapamil, 0.2 mg/kg, was given followed by an infusion of 3, 6, and 9 micrograms/kg per minute, which produced plasma verapamil concentrations ranging from 98 to 204 ng/ml. Verapamil significantly reduced the systemic vascular resistance index and mean arterial pressure. The decrease in afterload led to an increase in cardiac index, since there was little change in myocardial contractility (LV dP/dt). Administration of calcium chloride, 20 mg/kg, did not reverse the hemodynamic effects of verapamil. The data indicate that in dogs verapamil can be given, even in high concentrations, during fentanyl-nitrous oxide anesthesia without serious adverse electrophysiologic or hemodynamic consequences.