The development of mouse blastocysts transferred to the kidney and testis was studied by light and electron microscopy. Approximately 50% of the blastocysts transferred to both sites survived for more than five days. Although trophoblast proliferation was characteristic of all 31 successful transplants which were serially sectioned, inner cell masses differentiated in only nine. Transplants were invasive in kidney but were encapsulated in testis. In both organs, however, transplants produced considerable hemorrhage. That trophoblast invades by phagocytosis was not substantiated in this study nor was there evidence that trophoblast elaborates a cytolytic factor. Although transplants caused gross lesions in the host organs, the cytology of cells in close proximity to invading trophoblast was almost normal. The only abnormal features observed were the presence of fine fibrillar material in the basal cytoplasm of renal tubule cells and a thickening of their basement membranes. No correlation could be demonstrated between the antigenic dissimilarity of host and transplant and extent of host tissue response.