1. In 52 experimental animals (Ambystoma mexicanum) with a body length of 7.5-8.5 cm (including tail) the right telencephalon hemisphere was extirpated and substituted by transplantation of a hemisphere which was taken from an animal of the same bodylength. The heads of all experimental animals were cut in horizontal serial-sections. The result of the transplantation was compared with control animals (resection of one hemisphere without transplantation). 2. After transplantation marked degenerative changes occur in the transplanted hemisphere, but large parts of the matrix regions of the transplant remain intact. Moreover, differentiated brain tissue could be demonstrated at every stage. From the 4th day after transplantation numerous mitoses were found in the transplanted matrix tissue, which lead to an increase in the mass of the parts of the transplant that have remained intact. 3. On the 35th day after operation the transplant is in contact with the regenerating N. olfactorius for the first time. Between the 120th and the 160th day after operation the transplant reaches approximately the size of a normal hemisphere but the shape of the transplanted hemisphere remains very irregular even on the 450th day after operation. 4. By planimetrical investigation of the hemisphere cuts in experimentation and control animals and by statistic evaluation of measurements it could be proved that inAmbystoma mexicanum the resected hemisphere can be replaced by hemisphere-homotransplantation to a great extent during the postembryonal period.