Pregnant golden hamsters were injected intraperitoneally on the morning of the eighth day of gestation with a solution of copper citrate (2.7 mg/kg) or deionized water. Embryos were removed from the mothers by laparotomy on the 12th and 13th days of gestation, serially sectioned through the thorax, and examined for cardiac malformations. Twenty-one of the 37 edematous embryos from copper-treated mothers were affected with a total of 58 major cardiac anomalies. Sixteen of these embryos were affected with double outlet right ventricle in association with a membranous ventricular septal defect, two embryos contained hearts with overriding aortas, and three embryos were affected with isolated ventricular septal defects. Eleven of the 16 embryos with double outlet right ventricle were also affected with pulmonary trunk hypoplasia. The concept that these cardiac anomalies arise from a common pathogenetic basis is supported by the present model.