Leptin is a 16 kDa peptide produced by adipocytes in response to increasing fat stores and signals to the brain to stop eating and increase energy expenditure. Recent studies point out that the role of leptin is much broader and includes the regulation of reproduction. The lizard, Podarcis sicula, is an oviparous species characterized, at this latitude, by a seasonal reproductive cycle. Since in Podarcis sicula the recovery of the gonadal function coincides with the recovery of metabolic activity, we have hypothesized that leptin might be involved in the regulation of the reproductive function in this species. We have identified an immunoreactive band in the plasma of the female of Podarcis sicula, which comigrated with recombinant mouse leptin and cross reacted with polyclonal antibodies against mouse leptin. A RIA method developed by Linco (St. Louis, MO) was utilized to measure leptin concentration in plasma, liver and fat bodies of Podarcis sicula throughout the reproductive cycle. The antibody used in this kit (Linco's Multi-Species Leptin Radioimmunoassay kit) was produced in the guinea pig against human leptin but displays crossreactivity to leptin molecules of many species. The level of leptin in the plasma of Podarcis sicula was in the same range as that of mammals. Leptin levels in plasma, liver and fat bodies fluctuated during the reproductive cycle, in a way consistent with its possible role in reproduction.