2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related chemicals are potent cardiovascular teratogens in developing piscine and avian species. In the present study we investigated the effects of TCDD on murine cardiovascular development. Pregnant mice (C57Bl6N) were dosed with 1.5–24 μg TCDD/kg on gestation day (GD) 14.5. At GD 17.5, fetal mice exhibited a dose-related decrease in heart-to-body weight ratio that was significantly reduced at a maternal dose as low as 3.0 μg TCDD/kg. In addition, cardiocyte proliferation was reduced in GD 17.5 fetal hearts at the 6.0-μg TCDD/kg maternal dose. To determine if this reduction in cardiac weight was transient, or if it continued after birth, dams treated with control or 6.0 μg TCDD/kg were allowed to deliver, and heart weight of offspring was determined on postnatal days (P) 7 and 21. While no difference was seen on P 7, on P 21 pups from TCDD-treated litters showed an increase in heart-to-body weight ratio and in expression of the cardiac hypertrophy marker atrial natriuretic factor. Additionally, electrocardiograms of P 21 offspring showed that the combination of in utero and lactational TCDD exposure reduced postnatal heart rate but did not alter cardiac responsiveness to isoproterenol stimulation of heart rate. These results demonstrate that the fetal murine heart is a sensitive target of TCDD-induced teratogenicity, resembling many of TCDD-induced effects observed in fish and avian embryos, including reduced cardiocyte proliferation and altered fetal heart size. Furthermore, the combination of in utero and lactational TCDD exposure can induce cardiac hypertrophy and bradycardia postnatally, which could increase the risk of cardiovascular disease development.