SUMMARY Over a 9-month period, the incidence and characteristics of hypertension followtng coronary artery bypass surgery were studied in a group of 52 patients. Hypertension occurred in 61% of the patients and was characterized by an increase in arterial blood pressure of 35 ± 2 mm Hg mean ± SEM during the early postoperative period. Preoperative blood pressures and hemodynamk variables were similar in those who developed hypertension and those who remained normotenshe. Ninety-four percent of those who developed hypertension as compared to only 40% of those who remained normotensive received propranolol during the 24 hours preceding surgery (x1= 15.4;p< 0.001). Maximal blood pressures during the first 5 hours following the termination of cardiopulmonary bypass were significantly positively correlated with preoperative propranolol dosage (p< 0.01). Hypertension was not associated with significant changes in plasma renin activity or angiotensin II levels, but concomitant plasma catecholamine concentrations were elevated significantly (p< 0.005). However, a similar rise in plasma catecholamine concentrations was found in those who remained normotensive. Hypertension was associated with an increase in systemic vascular resistance (p< 0.001) and left ventricular stroke work index [p< 0.05), and a fall in stroke volume (p< 0.005) and cardiac index (p< 0.001). These studies suggest that hypertension following coronary artery bypass surgery is common, results from an increase in systemic vascular resistance, is not renin-angiotensin mediated, and may, in part, be related to preoperative propranolol administration.