The possibility that DL-carnitine has a protective effect during myocardial ischemia was evaluated by performing two rapid coronary sinus pacing studies 15 minutes apart in 21 patients with coronary artery disease. Eleven patients received DL-carnitine (20 or 40 mg/kg) before the second pacing study. The treated group had a significant increase in mean heart rate (12.5 beats/min, P less than 0.001), pressure-rate product (1,912 units, P less than 0.01) and pacing duration (3.2 minutes, P less than 0.001) after the administration of carnitine. The treated group also had improvements in percent myocardial lactate extraction (8.8 percent increase, P less than 0.001) and left ventricular end-diastolic pressure (a decrease of 5.3 mm Hg, P less than 0.05). There was significantly less S-T segment depression during the second pacing period in both the untreated and treated groups. The results of this study suggest that in ischemic human hearts with reasonably well preserved left ventricular function, DL-carnitine may improve the tolerance for stress associated with an increase in heart rate and pressure-rate product.