Timed pregnancies were induced in cats by synchronizing gonadotropin-stimulated estrus and ovulation with natural mating. The cats were given daily oral doses of thalidomide as follows: 10 mg/kg on days 10-20 of pregnancy, 240 mg/kg on days 10-14, and 120, 240, and 480 mg/kg on days 15-17 or 18-20 of pregnancy. Fetuses were delivered by cesarean section on day 44 or 45 or earlier if threatened abortion was considered imminent. A wide variety of cardiovascular anomalies was observed on gross and microscopic examination including the following ventricular septal defect, right atrial distension primarily involving the coronary sinus, malpositioned great vessels, and narrowed left ventricular chamber with hypertrophied walls. The overall incidence of these anomalies appeared related to dose and treatment period.