In the present study we observed a significant depression of thymocytes during pregnancy and investigated the influences of this thymic change on the immunologic capacities of peripheral lymphocytes. Thymocytes in pregnant mice began to decrease in number from Day 10 and reached about 0.1-fold of the nonpregnant level at Day 19, just before parturition. At late stage of pregnancy, thymocyte subpopulation expressing CD4+CD8+ and Thy1.2+PNA+ was selectively depressed. On the contrary, peripheral lymphocytes including splenocytes, peripheral blood lymphocytes and peripheral lymph node cells showed no depression. As to the immunologic capacities of the pregnant hosts, delayed footpad reaction and phagocytic activity of fixed liver macrophages in vivo were remarkably suppressed, but MLR reactivity and antibody response to SRBC or haptens were well preserved. Transfer of pregnant sera or administration with steroid hormones especially E3 into nonpregnant mice induced similar changes in the thymus and peripheral lymphocytes in number and subsets but this could not mimic the immunologic reactivities of the pregnant mice. These results suggest that sex steroid hormones such as E3 play an important role in the changes in cell populations of each lymphoid organ and the immune reactivities of the hosts during pregnancy. However, other factors also contribute to the immunologic capacities of the maternal hosts.