The aim of this study was to evaluate the effects of hyperleptinemia during the first ten days of life on thyroid function in adulthood. After birth, pups were separated into two groups: L8-receiving daily injections of recombinant mouse leptin (8 µg/100 g body weight, sc) and control (C)-receiving the same volume of saline. Both groups were treated for the first 10 days of lactation. The animals were sacrificed at 150 days of age, and the blood was collected for leptin, TSH, total triiodothyronine (TT 3) and total thyroxin (TT 4) serum concentration determinations by radioimmunoassay. The thyroid gland was excised to determine thyroid iodine uptake. Leptin, TT 3 and TT 4 serum concentrations in L8 group were significantly (108%, 47% and 32%; p< 0.05) higher than that of controls. There was no significant difference between the groups related to thyroid iodine uptake and TSH serum concentration. These data suggest that the first half of lactation period is important in determining thyroid function in adulthood, and that it can be programmed by serum leptin concentration.