The effect of a single oral dose of a plasma FFA-lowering drug (5-(3-pyridyl) tetrazole), which does not act by conversion into nicotinic acid, on exercise tolerance and ECG reaction was studied on a double-blind basis in 15 men with stable angina pectoris. Exercise was performed on a bicycle ergometer in the sitting position with a load increase of 10 W/min. In addition to ECG, time to onset of chest pains and to termination of exercise because of strong chest pains was recorded. 5-(3-pyridyl) tetrazole decreased plasma FFA during exercise from 523 to 299 mumol/l. It reduced significantly the ST depression at corresponding work loads and permitted the patients to exercise 0.6 min longer, corresponding to a 7% higher work load, before the onset of chest pain. However, absolute exercise time was not significantly increased. The most probable explanation of the improved performance is a decreased lipid and increased carbohydrate oxidation by the ischemic heart, although a contribution may have come from hemodynamic effects of the drug, unrelated to effects on myocardial metabolism but perhaps involving heart rate and BP. The lack of a significant effect on performance time may have been due to general fatigue.