Nitrofen [2,4-dichlorophenyl-P-nitrophenyl ether] is a diphenyl ether herbicide used for pre and post-emergent control of broad leafed weeds. This chemical was known to induce a variety of congenital cardiovascular anomalies with diaphragmatic hernia and hydronephrosis in the rate fetuses. The present study was conducted to produce congenital cardiovascular anomalies in the rat fetuses by oral nitrofen administration at the indicated doses and days of gestation, and to find the characteristics of nitrofen-induced cardiovascular anomalies. All the observed fetuses were removed from the pregnant Sprague-Dawley rats sacrificed on the twenty-first day of gestation. They were preserved in 10 per cent formalin and dissection for examination were carried out under a dissecting microscope using forceps and scissors. Following results and conclusion were based on dissecting microscopic findings on 482 offsprings. 1. The eleventh day of gestation was the most sensitive day for nitrofen induction of congenital cardiovascular anomalies in the rat. This incidence was dose-related in rats exposed on the eleventh day of gestation. 2. Ventricular septal defect was the most common single anomaly that represented more than half of the total cardiovascular anomalies, followed by aortic arch anomalies and tetralogy of Fallot. 3. Cardiac anomalies derived from infundibular maldevelopment such as tetralogy of Fallot and pulmonary atresia with ventricular septal defect were only observed in the eleventh gestation day treated group. 4. Aortic arch anomalies were found in high frequency and the great majority were characteristically anomalous right subclavian artery with left aortic arch. Key words; nitrofen, congenital cardiovascular anomalies.