This double-blind, randomized, crossover study examined the effects of intravenous infusion of esmolol (a new ultra-short-acting β-receptor blocking agent) and propranolol on cardiovascular performance at rest and during peak upright exercise in 15 patients. Biventricular function was assessed by means of first-pass radionuclide ventriculography with a computerized multicrystal camera. At rest, significant treatment differences between esmolol and propranolol vs baseline were found for the heart rate, systolic blood pressure, double product, left ventricular ejection fraction (EF), systolic blood pressure to end-systolic volume ratio, cardiac index and right ventricular EF. During exercise, significant treatment differences were also found for the heart rate, systolic blood pressure, double product, right ventricular EF and cardiac index. The mean baseline measurements were higher than the mean treatment measurements, but no significant differences were found between mean esmolol and mean propranolol measurements at rest and during exercise except for the exercise systolic blood pressure, which was lower during esmolol infusion. The magnitude of drug effect was greater at the time of exercise than at rest. The blood level of esmolol decreased markedly by 30 minutes after infusion. Esmolol was well tolerated, with no important local, systemic or laboratory abnormalities. Thus, the effects of esmolol on cardiovascular performance at rest and exercise are similar to those of propranolol. © 1985.