The gestational profiles of leptin in plasma, adipose tissue and placenta were investigated in rats. Leptin in plasma and the adipose tissue of the maternal compartment increased as pregnancy advanced and remained high during the latter half of pregnancy (approximately 2-fold compared with the non-pregnant state), followed by a rapid decrease just before parturition. Leptin in fetal plasma and amniotic fluid was first detectable on day 19 and then increased to levels comparable to those in maternal plasma of non-pregnant and day 21 pregnant rats. The total amount of mRNA in maternal adipose tissues significantly increased as pregnancy advanced and reached about 2.5-fold of those of non-pregnant rats on day 19 of pregnancy, followed by a marked decrease on day 3 of lactation. Placentae and decidual tissues did not show any expression of leptin mRNA on day 12 and 19 of pregnancy. These results indicate that the placenta is not a major source of leptin production in rats and also suggest the physiological significance of leptin in rat pregnancy.