L-Carnitine has been shown to improve the post-ischemic recovery of myocardial function and metabolic measurements that are reduced in the course of ischemia and reperfusion of the heart. In this study we used 40 male guinea-pigs in order to determine if the effect of L-carnitine which is used in the protection of the post-ischemic reperfused heart, is dose-dependent or not, All harvested hearts were perfused for 30 min on modified Langendorf apparatus with oxygenized Krebs-Henseleit solution. After this period, in (n = 10), 5 mmol and 10 mmol (group B. n = 10) of L-carnitine were added into a Krebs-Henseleit solution. After 20 min, perfusion was complete and the hearts were then exposed to normothermic ischemia for 20 minutes. Following the ischemia, hearts were reperfused with the same solutions for 30 min. In group C (n = 10), 10 mmol of L-carnitine was added into the solution at the postischemic reperfusion step. In the control group, the same procedures were performed without using L-carnitine, Matching was done according to the contractile force of the heart rate and the levels of malondialdehyde and adenosine deaminase, When 10 mmol L-carnitine was added into the perfusion solutions at the pre-ischemic period, the best results were obtained and myocardial damage was much less than the control group, The protective effects of L-carnitine in normothermic ischemia is dose-dependent and it must be given at the pre-ischemic period. (C) 1998 The International Society for Cardiovascular Surgery. Published by Elsevier Science Ltd. All rights reserved.