Parathyroid autografts were performed in 19 random bred mongrels and parathyroid allografts were exchanged between seven random bred mongrels. Three pairs of siblings from a beagle colony were immunosuppressed and parathyroid allografts exchanged between them. The parathyroid autografts were successful, while parathyroid allografts failed on all occasions in the non-immunosuppressed host. In immunosuppressed animals, however, the allografts were successful in five of six dogs. The success of parathyroid transplantation was determined by the following observations: 1) The experimental animal, having grafted parathyroid tissue as the only source of hormone, maintained a normal or near normal serum calcium concentration. 2) Following removal of the parathyroid graft, there was an immediate fall in the serum calcium concentration associated with tetany and/or death. 3) Histological study of the grafted gland revealed normal architecture, and 4) Radioimmunoassay of extracted grafts revealed moderate to large quantities of parathyroid hormone.