12 mongrel dogs were divided into two groups and anesthetized with 100% O and isoflurane in different concentrations (1MAC and 2MAC). Blood pressures and heart rates were monitored and cardiac outputs were measured during all experiments. Cardiovascular effects of isoflurane in different concentrations were investigated. And then, intra-venous verapamil was injected slowly in bolus (0.2mg/kg) in each group and its cardiovascular effects were also investigated. The results were as follows: 1) Heart rate was decreased when the concentration of isoflurane was increased (p<0.05) but there was no significant change after administration of verapamil. 2) Mean arterial pressure was decreased when the concentration of isoflurane was increased (p<0. 01), and after verapamil was injected intravenously in each group (p<0.01). 3) Cardiac index and stroke volume were decreased when the concentration of isoflurane was increased (p<0.05). 4) Left ventricular stroke work index was decreased when the concentration of isofiurane was increased (p<0.01) and it was also decreased significantly after administration of verapamil in high concentration of isoflurane (2 MAC) (p< 0.01), and the degree which was decreased by the administra- tion of verapamil was much more in high concentration of isoflurane than that of low concentration (p< 0.05). 5) There were no statistically significant changes in central venous pressure, pulmonary arterial pressure, pulmonary capillary pressure, vascular resistances, and right ventricular stroke work index.