Lactation-dependent regulation of leptin expression in mouse mammary gland and parametrial adipose tissue was estimated by RT-PCR analysis for virgin, pregnant, lactating and post-lactating mice, and the serum and milk leptin levels of these mice were also determined by ELISA. Leptin gene expression in mammary gland as well as in adipose tissue was obviously detected before pregnancy, markedly decreased to 30-50% after parturition and kept at the low level during lactation period, and restored to the original level after weaning. The leptin concentration of milk collected just before weaning was about two-fold higher than that of the milk collected at mid-lactating stages. The serum leptin levels of the mid- and late-lactating mice were not significantly higher than those of non-pregnant mice. These results suggested that the lactation-induced down regulation of leptin was associated with autocrine/paracrine action of leptin in mammary and adipose tissues, and that the milk leptin, especially at the latter stages of lactation, was not only ascribed to diffusive transport from maternal blood stream, but also regional production and secretion by mammary epithelial cells. This possible production of leptin by mammary epithelial cells was further supported by the fact that leptin was expressed by cultured cells of mammary epithelial cell line, COMMA-1D, in a manner negatively dependent on the lactogenic hormones.