In order to evaluate the clinical efficacy on penbutolol (Hoe 893 d), a new adrenergic blocking agent, two groups of 10 patients (A & B) with ischemic heart disease were chosen. These were submitted to exercise testing for 3 minutes, and placebos, nitroglycerin, and propranolol were used as reference preparations. As an index of indirect consumption of oxygen we determined the maximum cardiac frequency (MCF), maximum systolic arterial pressure (MAP) and the maximum double product (MDP). An electrocardiographic control of the patients was also included to observe ST segment changes. The patients from group A received 0.8 mg. of nitroglycerin sublingual as the first treatment and 40 mg. of propranolol as the second. The patients from group B received initially a placebo P.O. and then 25 mg. of penbutolol P.O. In both groups the exercise testing was repeated 10 minutes after the administration of the first substance and 10 minutes after the second. The results showed that 25 mg. of penbutolol and 40 mg. of propranolol have similar therapeutic effects over the degree, intensity, and duration of the differences in elevation of the ST segment, MCF, MAP, and MDP when you compare the results of their own controls with those obtained with the active principle. There is no significant difference observed in the results obtained in each parameter studied between penbutolol and propranolol. During the period of this clinical study there were no undesirable collateral effects caused by the oral administration of the investigated drugs.