The effects of three different plasma levels of verapamil on coronary hemodynamics and myocardial metabolism in the presence of 1.61 ± 0.05% end-tidal concentration of isoflurane (mean ± SEM) were studied in a canine model, using a thermodilution coronary sinus catheter to measure coronary sinus blood flow and pressure and to provide coronary sinus plasma samples. A control group receiving only isoflurane was also studied (n = 6). Plasma arterial verapamil levels of 55 ± 7 (n = 6); 134 ± 7 (n = 10); and 301 ± 37 ng · ml−1(n = 5), were achieved by a loading dose followed by a continuous infusion for 30 min. The only changes with time in the isoflurane group were decreases in left ventricular maximum rate of tension development (dP/dt) and left ventricular stroke work index compared with control after 90 min without changes in myocardial oxygen balance. The low plasma verapamil level caused reductions in heart rate, mean and diastolic arterial pressure, and left ventricular dP/dt without changes in myocardial oxygen supply or myocardial metabolism. Intermediate verapamil concentrations produced a transient initial increase in heart rate and a reduction in stroke volume index. With the intermediate and the highest levels of verapamil, mean and diastolic arterial pressure, left ventricular dP/dt, and cardiac index were decreased. An increase in arterial norepinephrine plasma levels was seen in the intermediate and the highest levels of verapamil; however, a transient coronary vasodilation occurred without changes in myocardial oxygen balance. Significant prolongation of the PR interval was observed in all verapamil groups, with second or third degree heart block in some of the higher-dose animals. Despite no adverse effects on myocardial oxygen balance when these concentrations of isoflurane and verapamil were combined, conduction block at moderate plasma levels may limit the usefulness of verapamil during isoflurane anesthesia.