Haemodynamic and plasma catecholamine responses were evaluated during isoflurane anaesthesia in ten patients undergoing coronary artery bypass surgery. Following thiopentone induction the patients were anaesthetized with isoflurane 1.5-2.0 per cent in oxygen. The results show that after 10 minutes of isoflurane anaesthesia there was a significant increase from baseline in heart rate, 68 to 80; cardiac output, 3.75 to 4.61; and plasma epinephrine, 0.80 to 1.33 microgram/l. Conversely, there was a significant reduction in systemic vascular resistance index, 3388 to 2260, and plasma norepinephrine, 1.10 to 0.88 microgram/l. Twenty-five minutes later, after sternotomy, heart rate, cardiac output and the level of plasma epinephrine were still elevated, and systemic vascular resistance index and plasma norepinephrine remained lowered (p less than 0.05). This study demonstrates significant catecholamine responses during isoflurane anaesthesia. The increase in plasma epinephrine parallelled the increase in heart rate and cardiac output, and the decrease in plasma norepinephrine paralleled the decrease in systemic vascular resistance. Based upon these findings we conclude that catecholamine responses contribute to the cardiac and peripheral cardiovascular changes observed with isoflurane anesthesia.