The influence of carnitine on male reproductive function is well established. However, the molecular mechanism by which carnitines control male fertility have not yet been clearly identified. In the present study, we studied the role of carnitines at testis level by examining their effects on protein metabolism in cultured rat Sertoli cells. Our results demonstrate that L-carnitine and, in particular, L-acetylcarnitine administration affects overall protein synthesis, inducing a dose-dependent decrease in amino acid incorporation and consequently an increase of the intracellular amino-acid pool useful to speed up Sertoli cell energy metabolism. We also observed significant effect on the expression of specific proteins involved in the regulation of glucose uptake and Insulin-like Growth Factor (IGF) biological activity. Actually, an increased expression of the mRNA coding for the membrane Glucose Transporter-1 (GLUT-1 ) and a decreased expression of mRNA coding for the negative modulator insulin-like growth factor-binding protein-4 (IGFBP-4) were induced by in vitro treatment with L-carnitine and L- acetylcarnitine separately and in combination; in fact, the effects observed were far more pronounced following concomitant treatment with both compounds. Taken together, these data indicate that the influence of carnitines on male fertility could be partly mediated by an activity at Sertoli cell level.