Dibenzo[def,p]chrysene (DBC) is a transplacental carcinogen in mice (15mg/kg; gestation day (GD) 17). To mimic residual exposure throughout pregnancy, dams received four smaller doses of DBC (3.75mg/kg) on GD 5, 9, 13 and 17. This regimen alleviated the previously established carcinogenic responses in the thymus, lung, and liver. However, there was a marked increase in ovarian tumors (females) and hyperplastic testes (males). [(14)C]-DBC (GD 17) dosing revealed transplacental distribution to fetal tissues at 10-fold lower concentrations than in paired maternal tissue and residual [(14)C] 3weeks post-dose. This study highlights the importance of developmental stage in susceptibility to environmental carcinogens.