Evidence has been put forth that a number of human and experimental cardiomyopathies are associated with a lower myocardial carnitine content. This study was performed to test the hypothesis that the correction of carnitine deficiency by a naturally occurring carnitine derivative, propionyl-L-carnitine (PLC), may improve cardiac function. Repeated administration of PLC was compared to saline with respect to cardiac function in rats with pressure-overload cardiac hypertrophy and low myocardial carnitine levels. Cardiac hypertrophy was induced by abdominal aorta constriction in rats. Separate groups of rats were used for (a) determination of myocardial carnitine content, (b) evaluation of in vivo hemodynamics, and (c) evaluation of performance and metabolic state of Langendorff perfused hearts. Results showed the following: (i) The myocardial carnitine content was inversely correlated to cardiac hypertrophy (r = 0.68, p < 0.05) and PLC treatment (50 mg/kg i.a. for 4 days) restored it to normal values (ii) The PLC effect on cardiac function was significantly and directly related to cardiac hypertrophy [correlations between heart weight and percent changes in cardiovascular parameters: cardiac output (CO), p < 0.001; cardiac work (CW), p < 0.01, stroke volume (SV) and stroke work (SW), p < 0.02]. In animals with heart weight > 1,400 mg, the effect of PLC on CO, CW, SV, SW, and total peripheral resistance (TPR) was significantly different from that of saline (CO, CW, SV, and SW, p < 0.005 each; TPR, p < 0.05). The effect was observed 24 h after the first PLC administration and significantly diminished following a 4 day suspension of the treatment, (iii) Perfused hearts from PLC-treated rats displayed a significantly lower left ventricular end-diastolic pressure (p < 0.01) and greater relaxation rate (p < 0.05) than those from control rats. Moreover, in PLC-treated hearts, the content of creatine phosphate, ATP, and total adenine nucleotides (ATP + ADP + AMP; TAN) was significantly increased (CP, p < 0.05; ATP and TAN, p < 0.01 vs. control). These data show that PLC exerts a stimulatory activity on hearts with hypertrophy and low carnitine content, implying that carnitine deficiency may contribute to the depression of cardiac function in this model.