Expts. are reported on young dogs between 3 and 8 wks. of age who were subjected to end-to-end anastomosis of the thoracic aorta. Studies were completed on 3 dogs that survived approx. 1 yr. Narrowing of the aortic lumen at the suture line was demonstra- ted in each animal by angiocardiographic methods, and in one by thorocotomy. The expts. may be said to have some bearing on when aortic anastomosis may best be done in resection of coarctation of the aorta. They suggest that the diam. of the lumen at the site of the anastomosis in these dogs did not keep pace in growth with the rest of the aorta. The fact that a single continuous suture was used for the anastomosis may have some bearing on the formation of the constriction, but even so, these expts. tend to support the view that surgical intervention for coarctation should be deferred until the lumen of the aorta is large enough in diam. to insure adequate size as the individual grows to maturity.