Changes in adrenal carbonyl reductase (CR) activity during pregnancy and postparturition in rats were investigated using two substrates, 13,14-dihydro-15-keto-prostaglandin F2 alpha (15KD-PGF2 alpha) and p-nitroacetophenone (PNAP). 15KD-PGF2 alpha reductase activity showed the lowest level on day 16 of pregnancy and steeply increased on the morning of postpartum day 1. PNAP reductase activity did not change up to day 18 of pregnancy, then was gradually increased and maintained the highest levels between day 19 and the morning of day 21 of pregnancy. Thereafter, the enzyme activity was rapidly decreased after parturition, on the contrary of changes in the 15KD-PGF2 alpha reductase activity. On the other hand, exogenous glucocorticoid treatment increased the 15KD-PGF2 alpha activity and decreased the PNAP reductase activity in adrenal glands of nonpregnant animals. Thus, it is found that the adrenal CRs in the above two classes are regulated by different mechanisms. As maternal corticosterone level is increased rapidly during late pregnancy by maternal transfer of fetal corticosterone, the present results suggest that adrenal CR activity during pregnancy, in particular at periparturition, may be influenced by fetal corticosteroids.