2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes thymic atrophy as well as alterations in thymocyte maturity in mice. Multiple mechanisms for thymic hypocellularity have been suggested, and include an increase in thymocyte apoptosis, a maturation arrest of thymocyte development, inhibited thymocyte proliferation, and a diminution of seeding of the thymus by the hematopoietic progenitors in the fetal liver or adult bone marrow. Fetal mice are highly sensitive to hypocellularity induction by TCDD when the chemical is administered during the window of thymic development, between days 10 and 18 of gestation. Treatment of pregnant C57Bl/6 mice in the present experiments with doses of 5 or 10 mu g/kg TCDD by oral gavage on gestation days 14 and 16 severely depressed day 18 thymic cellularity. Histopathologic evaluation of day 18 fetal thymi showed disruption of the normal organ architecture with loss of clear distinction between cortical and medullary regions after TCDD. A decrease in thymocyte density was noted in all regions, and was most dramatic in the cortical zones where pyknotic cells were increased by TCDD treatment. Using day 18 thymocyte suspensions and flow cytometry, the marker 7-AAD showed a decrease in viable thymocytes from TCDD-treated fetal mice, and a concomitant and dose-related increase of thymocytes in early apoptosis. Specifically, relative to control, thymocytes from the 5 and 10 mug/kg TCDD exposure groups displayed 1.9% and 5.3% respective increases in early apoptotic cells. When thymocytes were co-identified by CD4 and CD8 cell surface antigen expression, the enhanced apoptosis occurred in the CD4(+)CD8(+) phenotype with no significant apoptosis seen in the CD4(-)CD8(-), CD4(+)CD8(-), or CD4(-)CD8(+) thymocytes. Given the rapid clearance of apoptotic cells from the thymus, these histopathologic and cytometric data suggest increased thymocyte apoptosis contributes to fetal thymic atrophy after TCDD exposure.