The teratogenic effects of propranolol HCl on cardiac development were studied in chick embryos of days 3 and 4 of incubation. Propranolol was injected into the yolk sac at doses ranging from 0.05 to 0.6 mg per egg. All the treated and control embryos were examined on day 7. The LD50 for the embryos treated on the 3rd and 4th day was 0.15 and 0.35 mg per embryo, respectively. Cardiac anomalies such as aortic stenosis ventricular septal defects and common truncus arteriosus were observed. Other malformations included atrial septal defects, thin atrial wall and defects of the pulmonic, aortic and atrioventricular valves. The incidence of cardiac anomalies in the controls was very low. Propranolol was observed to slow the heart rate in the experimental embryos. It is suggested that slowing of heart rate at the early stages of heart development caused aberrant bloodstream flow patterns which probably resulted in the genesis of cardiac anomalies. The results of this study indicate that propranolol has teratogenic effects on chick embryo cardiogenesis.