The tetralogy of Fallot was produced experimentally in the chick embryo by altering the development of the conus and delaying the disappearance of the conoventricular groove. The experimental procedure used consisted of placing a ligature or cerclage around the proximal portion of the conus for 24 hr in embryos in Hamburger-Hamilton stages 17–21, thus altering cardiac hemodynamics particularly in the clonal region. At the same time the conus was prevented from aquiring its normal position owing to the persistence of the conoventricular groove. As a result, the most frequently observed abnormality (50%) was a complex of anomalies similar to tetralogy of Fallot in man. Varying degrees of pulmonary stenosis were observed in 85% of the experimental cases either as the only defect or in association with other cardiac malformations.