In rat hearts perfused using the Langendorff technique, a cellular release of myoglobin (an index of sarcolemmal damage) was induced in a dose-dependent way by palmitoyl carnitine concentrations exceeding 1.6 microM in the perfusion solution. From 0.7 to 64.5 mmoles palmitoyl carnitine/kg dry wt were taken up by the heart tissues exposed for 30 min to extracellular palmitoyl carnitine concentrations ranging between 0.7 and 100 microM. The steep S-shaped curve relating the cellular myoglobin release provoked by Ca2+ readmission after Ca2+-free perfusion (the calcium paradox) to the perfusion temperature was shifted to lower temperatures by 2 to 3 degrees C in the presence of 1.6 microM palmitoyl carnitine. The loss of myoglobin induced by a mechanical distention of the left ventricular wall during Ca2+-free perfusion was nearly doubled in the presence of 1.6 microM palmitoyl carnitine. Isolated rod-shaped myocytes turned round within 20 min when the extracellular palmitoyl carnitine concentration exceeded 1.6 microM. It is concluded that the presence of palmitoyl carnitine in the perfusion medium at concentrations below those usually adopted in the literature to study the effects of palmitoyl carnitine on the sarcolemmal function induces membrane disruption and exacerbates the membrane damage caused by other factors. The tissue amphiphile content is probably critical to these effects.