Paroxysmal hypertension after cardiac surgery is a phenomenon of physiological and clinical significance. The possible preoperative and intraoperative factors that may predict its occurrence were studied in 81 consecutive patients undergoing coronary artery surgery (n = 58) or valve replacement (n = 27), of whom 45 (56%) developed postoperative hypertension. Hypertension occurred significantly more often in those patients who received beta adrenergic blocking agents preoperatively and who underwent coronary artery surgery. Patients with hypertension had significantly higher mean left ventricular ejection fractions preoperatively (52%) than those without (41%) and required phentolamine significantly more often and isoprenaline significantly less often intraoperatively. It is suggested that the significance of preoperative beta adrenergic blockade, the type of operation, and the intraoperative requirement for phentolamine in patients who developed post-operative hypertension may indicate the role of enhanced sympathetic activity and disturbance of cardiac receptors during surgery. Preoperative myocardial performance and the method of myocardial protection during surgery are likely to influence the occurrence of the hypertensive phenomenon.