Radiation of 2 MeV neutrons was used to induce conotruncal anomalies experimentally in chick. White leghorn eggs were exposed to a single dose of neutrons ranging from 50 to 250 rads at various stages of the development. Cardiovascular anomalies were found in 209 (40%) of 526 treated embryos; conotruncal anomalies (81/209 or 39%), simple VSD (56/209 or 27%), isolated aortic arch anomalies (69/209 or 33%) and others (3/209 or 1%). The conotruncal anomalies were induced at considerably high incidences by exposures during the 3rd day of incubation and the highest incidence was 74% in the cases malformed by 220 rads. The types of conotruncal anomalies observed were as follows: VSD with pulmonary overriding (52 cases), VSD with aortic overriding (11 cases), DORV (10 cases), truncus arteriosus (6 cases) and complete TGA (2 cases). Sixty (74%) of these cases had aortic hypoplasia, constituting coarctation or interruption complex similar to that seen in man. It is generally thought that the experimental production of complete TGA in chick appears to be impossible when utilizing ordinary teratogenic means. However, neutron radiation could induce this peculiar anomaly in chick.