Establishment of bypass using free grafts has advantages over STA-MCA anastomosis in securing sufficient blood supply to the intracranial arteries. Arterial grafts irradiated with high energy electron are known to be definitely aseptic and to evoke little tissue reaction and this method seems to be best suited for utilization of arterial grafts from human cadaver.<BR>Experimental studies were performed using, first, the homologous arterial grafts 5 cm in length for assessment of the long-term results in dogs, secondly, the homologous grafts 10 cm in length for establishment of a bypass through the tissue planes in dogs, thirdly, the homologous grafts of a small caliber, less than a millimeter, for feasibility of a small arterial bypass in rats, and lastly the heterologous grafts. These grafts were prepared by irradiation of 2,000,000 rads of electron generated by Van de Graaff accelerator and had been stored at 25°C until use. A segment was resected from the carotid artery and replaced with these grafts by anastomosing end-to-end. The patency of the grafts were examined angiographically and gross and histological examinations were made at various intervals.<BR>Observations from twenty-four hours up to two years after transplantation of the homologous grafts in fourty-three dogs revealed an excellent patency rate; only one had been occluded when it was examined on the third day. On histological examinations the structure of the grafts had been well preserved for a long time, especially of the elastic fibers; remarkably few changes were noted in it even two years after transplantation. The high patency rate may be attributed not only to very little histological changes for a long time period, but also to adoption of the microsurgical technique in the anastomosing procedure.<BR>Experiments using the long arterial grafts were carried out in 3 dogs and these grafts were made around the sternocleidmastoid muscle as a loop, simulating a tortuous course through the tissue planes. They had been patent in all three cases for 2.5 months period, showing, though too small in number to draw a definite conclusion, that irradiated arterial grafts could be used to bridge over a certain distance.<BR>Experiments using the homologous grafts of a small caliber were done in 8 rats and five grafts were patent and the other three grafts were occluded probably due to the technical failure. These experimental results indicate that even very small sized arteries could be substituted with the arterial graft irradiated with high energy electron.<BR>Experiments using the heterologous grafts were designed in three combinations; the rabbit aorta to the carotid artery of a dog in 10, the pig intercostal artery to the carotid artery of a dog in 7, and the rabbit carotid artery to the carotid artery of a cat in 3. Only 7 grafts of the rabbit aorta transplanted into a dog were patent; of these 7 patent grafts six were greatly expanded to form an aneurysm. Histological examinations showed that the elastic fibers of the grafts were destroyed in an early stage and replaced by a fibrous tissue. There was, however, no cell infiltration within the graft in seven patent grafts. On the other hand, massive cell infiltration was noted in all occluded heterografts. From these results it may well be concluded that high energy electron irradiation could not prevent or suppress tissue reaction to the heterologous grafts.<BR>It is concluded that, as far as the homologous arterial grafts are concerned, high energy electron irradiation gives a promissing outlook for construction of a bypass over the occluded cervical carotid artery by means of transplantation.