We investigated the effect of L-carnitine on the recovery of cardiac function after ischemic arrest in the perfused rat heart. L-carnitine was added to a cardioplegic solution, both as a free base and hydrochloride. The addition of L-carnitine as a free base to the solution had no effect on recovery of cardiac function. When L-carnitine HCl was added to the cardioplegic solution, it was necessary to adjust the pH of the solution to 7.4. The hearts arrested with this solution showed a greater incidence of reperfusion dysrhythmias than those in the control or the free base solution, but the overall recovery of cardiac function was the same as control. The hydrochloride of L-carnitine is strongly acidic, and these findings indicate that either the free base or a properly buffered solution must be used to study effects of carnitine upon cardiac function.