Syngeneic and allogeneic parathyroid gland transplants to the uterus of pseudopregnant and non-pseudopregnant, parathyroid-ectomized rats were performed. The functional survival of these grafts was assessed by determination of subsequent serum calcium levels. Histological evidence revealed that implantation took place in such a way that the uterine epithelium covered the ectopic parathyroid. Implantation directly upon a site where decidual proliferation had been produced did not occur. Syngeneic parathyroid grafts to the uterus functioned indefinitely throughout many estrus cycles. Allogeneic transplants of parathyroid glands to pseudopregnant and non-pseudopregnant uteri were almost uniformly rejected by 20 days following transplantation. Histological evidence was compatible with typical homograft rejection.