Weanling and adult rats were intraperitoneally injected with D-carnitine for 40 days. After 15 days of treatment a statistically significant depletion in the muscle levels of free and total L-carnitine was observed, but after 40 days this depletion became slight and statistically not significant. These findings suggest the presence of a compensatory mechanism acting on the inhibition of L-carnitine transport into skeletal muscle fibres.