Intraoperative hypertensive episodes are a common problem in patients undergoing coronary artery bypass grafting. Twenty patients who developed acute hypertension (mean arterial pressure increase to 110 mmHg) were studied. Ten patients received nifedipine (about 3 micrograms kg-1 min-1) and ten patients nitroprusside (about 0.75 micrograms/kg-1 min-1) to return arterial blood pressure to control levels. All patients were anaesthetized with flunitrazepam, fentanyl, pancuronium and N2O/O2. The study compares the effects of nifedipine and nitroprusside on systemic and pulmonary haemodynamics. Both nifedipine and nitroprusside decreased arterial pressure to baseline values within about 3 min by reducing the elevated systemic vascular resistance. Cardiac filling pressures and pulmonary artery pressure decreased significantly only with nitroprusside. Following nitroprusside cardiac output remained unchanged whereas nifedipine increased cardiac output and stroke volume when blood pressure was lowered by a comparable degree. The data suggest that nifedipine primarily affects resistance vessels in the systemic circulation without significantly changing venous tone as opposed to the effect of nitroprusside. Thus, nifedipine appears to be an appropriate vasodilator for controlling arterial hypertensive episodes in patients with coronary artery disease and normal left ventricular function.