Leptin encoded by the obese gene exhibits various functions, especially in the regulation of food intake and energy expenditure. The aims of this study are to investigate some specific intestinal roles of leptin, i.e. the regulation of epithelial cell proliferation and the nitric oxide (NO) production in the small intestine from rats. A total of 32 male, 3 month old, Swiss albino rats were divided into 4 equal groups: animals received a single intraperitoneal injection of the recombinant leptin (200 mu g/kg) in the group 1 and of N-nitro-L-arginine methyl ester (L-NAME) (30 mg/kg), a nitric oxide synthase (NOS) inhibitor, in the group 2. Rats of the group 3 were treated by L-NAME (30 mg/kg) 15 minutes before the leptin injection (200 mu g/kg) and, in the group 4, rats received saline and served as controls. One hour after the last injection, blood samples were collected for the determination of plasma NO concentrations. After slaughtering, small intestines were harvested and treated for histological observations and immunohistochemistry in order to evaluate NOS expression and cell proliferation via proliferating cell nuclear antigen (PCNA) immunostaining. Significant morphological changes of epithelial cells evidencing by enlargement of cellular height and a marked increase of epithelial cell proliferation compared to the controls were induced by treatment with leptin alone or in combination with L-NAME. Furthermore, in leptin-treated rats, endothelial nitric oxide synthase (eNOS) synthesis was enhanced in goblet cells from the Lieberkuhn glands leading to a slight increase of plasma NO concentrations whereas inducible nitric oxide synthase (iNOS) expression remained unchanged. Although L-NAME alone or injected before leptin depressed plasma NO concentrations, modifications of epithelial cell chacteristics, a strong intensity of epithelial cell proliferation, as well an increased eNOS expression were also observed in the groups 2 and 3. These results demonstrate that leptin acts as a mitogene factor on epithelial cells of the small intestine and would have some medical indications. But even if eNOS was up-regulated in parallel, the molecular mechanisms leading to cell proliferation seem to be NO independent.