Isoflurane is known to produce slight tachycardia in humans. This study examined the effects of isoflurane on cardiovascular parameters in dogs. Four groups, with six dogs per group, were anesthetized with isoflurane. Prior to isoflurane administration, a femoral artery catheter was inserted. Group 1 was anesthetized with isoflurane alone. Group 2 was pretreated with fentanyl prior to administration of isoflurane. Group 3, anesthetized with isoflurane alone, had a Swan-Ganz catheter introduced through the external jugular vein. Group 4 was pre-treated with fentanyl prior to administration of isoflurane, and had a Swan-Ganz catheter. Physiologic parameters were recorded at 15-min intervals as isoflurane was reduced from 3.5% to 1.5% by 0.5% increments. Heart rate increased while blood pressure decreased during induction (8.5 min) in Group 1 and then returned to control values. In Group 2, heart rate declined with no changes in blood pressure over all isoflurane concentrations. The induction time (time from initiation of the anesthetic until intubation was achieved) was 2 min. In Group 3, the heart rate increased and the blood pressure decreased, with an induction time of 10 min. Cardiac output and pulmonary artery pressure varied inversely to the isoflurane concentration. In Group 4, heart rate decreased with a minimal decrease in blood pressure, and an induction time of 3.5 min. Cardiac output and pulmonary artery pressure varied inversely to the isoflurane concentration. A fifth group of 6 dogs was monitored for heart rate only, while a mask was placed over their noses to simulate the procedure for the administration of an anesthetic. The heart rate increased similar to that of the dogs in Groups 1 and 3, but the tachycardia was abolished with the administration of fentanyl. Increased heart rate could not be directly attributed to isoflurane but was probably due to catecholamines released during induction. Fentanyl blocked this effect, resulting in a decrease in heart rate.