Tissue molds consisting of polyester mesh jackets covering silicone mandrils were implanted subcutaneously in dogs. When a sufficiently strong layer of connective tissue had formed around the molds they were excised and the mandrils removed. The process of tissue formation was observed and recorded. The forms were used as grafts in either the abdominal aorta or the femoral artery of the dog. Six grafts 10 mm inner diameter, six grafts 6 mm inner diameter and 26 grafts 3 mm inner diameter were implanted. After 4.5 months 80% of the 10 mm grafts, 83% of the 6 mm grafts, but only 4% of the 3 mm grafts remained patent. Histological studies at various intervals after grafting revealed gradual thickening of the neointima. The reason for graft failure was thrombus formation originating at the line of anastomosis.