Given the deregulation of leptin during catch-up growth period and the reduction in leptin resistance in old rats by carnitine supplementation, the purpose of this study was to determine serum leptin concentrations in rats receiving protein or energy restricted diets and the effects of carnitine supplementation on growth rate and leptin secretion. A total of 72 Sprague-Dawley rats, 6 week old, were randomly allotted in 3 equal groups (each containing 12 males and 12 females) according to the diet regimen: a 4 weeks long restriction period in which rats received a protein deficient ration (group PR) or in energy deficient ration (group ER) or a basal diet (controls) was followed by it 4 weeks long catch up period in which all rats were nourished with the basal diet and half of them in each group was orally supplemented with L carnitine (500 mg/kg/day). Body weight and food intake Were measured weekly and serum leptin concentrations were measured on days 0, 28 and 56. At the end of the restriction period, the body weights and the body weight gains were significantly higher in male and female controls than in both dietary restricted rats following the catch up period, a significant increase of the body weight gains calculated between the days 28 and 56 was evidenced in restricted males and females compared to the corresponding controls leading to similar final body weights in females whereas males from the groups PR and ER exhibited significantly lower final weights than control males. In parallel. the leptin concentrations and the relative variations of the leptin concentrations for the restriction period were significantly depressed in dietary restricted males compared to male controls, whereas no significant variation between controls and restricted rats was evidenced during the catch Lip period. Although body weights and leptin concentrations positively correlated on day 0 in the 3 groups, no significant association was obtained in malnourished rats on day 28 whereas this correlation was restored on day 56 in the PR group. Furthermore, the L carnitine supplementation has not significantly modified the leptin concentrations although it has significantly increased body weights and weight pills in control and protein restricted males. These results Suggest that the leptin secretion was independent of the L carnitine administration and was not exclusively regulated by the adiposity degree and that L carnitine would improve growth rate in protein depleted rats.