The fate of solid tissue allografts--skin or parathyroid glands--implanted in the interstitial tissue of the testis was investigated using inbred rats. The results affirm that the testis is an immunologically privileged site despite its efficient lymphatic drainage. Skin allografts survived at least several days longer than orthotopic grafts of similar size, whether major histocompatibility complex (MHC)-compatible or MHC-incompatible, and failed to induce an alloantibody response in most recipients or to prime for secondary antibody responses on rechallenge. Further assessment employed parathyroid grafts that allowed appraisal of their function and its duration, by monitoring serum calcium levels. Most intratesticular MHC-incompatible parathyroids survived for at least twice as long as control grafts in nonprivileged sites, with a median survival time (MST) of 41 days--and one-third of the grafts functioned at 100 days. MHC-compatible grafts fared even better (MST of 60 days), some surviving more than 400 days. Most F1 hybrid grafts survived virtually indefinitely. Splenectomy 5-23 days prior to implantation had a beneficial rather than a detrimental effect on the privilege afforded intratesticular parathyroid allografts. Allograft rejection was accompanied by antibody production in only one-half the animals. Grafts that had undergone functional rejection at the time of recovery usually had an intense mononuclear cell infiltrate, and long-term surviving grafts displayed varying degrees of cellular infiltration among cords of healthy, functional chief cells. Accepted parathyroids were destroyed by active immunization of the host but were unaffected by passively administered alloantibodies. The possible mechanisms controlling graft rejection in this unique privileged site are discussed.