Free autologous muscle transplants were performed on 59 Wistar rats of 70 to 90 days of age according to the method described by Schmidt (1978). The free smooth muscular transplant was prestretched by 0%, 100%, 120%, 150% and 200% and the animals were sacrificed on the 5th, 10th, 20th, 40th and 60th postoperative day, respectively. Five animals died postoperatively, of these three during ileus and stenosis after more than 150% prestretching of the transplant. Complications occurred in eleven further animals, which included again ileus phenomena nine times due to stenosing or disturbance of defecation, also as a result of obstruction phenomena. Complications always occurred in animals with more than 150% transplant prestretching. No relaxation reflex was observed manometrically in any of the transplants. Budding of nerve fibres also did not occur in any of the cases, neither from the plexus myentericus of the inner intestinal wall nor from outside. Histology revealed already after five days' survival leucocytous infiltration and connective tissue multiplication which increased with increasing prestretching of the transplant and longer survival time of the animals until complete metaplasia of the connective tissue. In preparations with 100 up to 120% prestretch not more than 20% of the original muscular mass was preserved. With prestretching of 150% and more the transplant underwent complete fibrosis. On the basis of these studies we must discourage performance of free transplants of smooth-muscle colon segments in newborn and young infants.