The serum concentration of leptin in 10 week old virgin ICR mice assessed by RIA was 1.70 +/- 0.08 ng/ml. The serum leptin concentration in the pregnant mice mated at 10 weeks of age significantly increased from day 11 of pregnancy and reached a peak on day 17 of pregnancy (42.2 +/- 4.8 ng/ml). AFter the delivery, the serum leptin concentration rapidly decreased and reached the level of the virgin mouse on the seventh day in the puerperium. Tissue contents of leptin in the placenta, the decidua, the uterus, and the adipose tissue were between 40 to 130 ng/g wet tissue. However, leptin mRNA was expressed only in the adipose tissue and the level of leptin mRNA on days 13 and 17 of pregnancy increased 3- to 5-fold compared with that of virgin mouse. Tissue content of leptin in the adipose tissue significantly increased from day 17 of pregnancy compared with that of the virgin mouse. The m-leptin secretion from the adipose tissue also significantly increased in vitro. These results suggest that leptin, which was secreted by adipose tissue, may play important roles in mouse reproduction after midpregnancy.