The effects of a serotonin (5-HT) precursor 5-hydroxytryptophan (5-HTP) on serum leptin levels were investigated in mice. 5-HTP dose dependently increased serum leptin levels in mice. Pretreatment of the peripheral aromatic amino acid decarboxylase inhibitor carbidopa suppressed 5-HTP-induced hyperleptinemia. These results suggest that the secretion of leptin may be modified by serotonergic mechanisms.