SUMMARYA technique of intrauterine renal transplantation in the fetal lamb was developed. Fetal renal allografts were implanted into the neck of a fetus in a second pregnant ewe. Donor and recipient gestational age varied from 70 to 130 days. Gestational age, operative exposure time, and transplant ischemia time were found to be important factors in determining technical success. Three to four days following transplantation, only a minimal perivascular infiltrate was seen in the allograft, and kidneys excreted urine with relatively normal osmolality and sodium content. Rejection was complete 7-9 days following transplantation. A proliferation of mononuclear cells, many pyronin-positive, occurred in the germinal centers of the recipient spleens concurrently with rejection. Thus the developing fetal lamb, during the latter half of gestation, can reject a vascularized allograft with the rapidity and intensity of an adult animal.