The testis is an immunologically privileged site, and transplantation data suggest that this privilege may be enhanced in cryptorchidism. Although alphabetaT cells, which mediate and promote the immune response, have access to the normal testis, relatively little is known about these cells in the abdominally located testis. An increase in testicular lymphocyte-inhibiting cytokines has also been implicated in enhanced graft survival following the experimental induction of cryptorchidism. Consequently, T cell traffic and lymphocyte-inhibiting activity in testes of cryptorchid adult rats were examined in the following study. Numbers of alphabetaT cells and the cytotoxic CD8(+) T cell subset in the testis were unaffected following 1 month of cryptorchidism. In contrast, subcutaneous testosterone implants, which inhibit Leydig cell function through suppression of gonadotrophin secretion, reduced these parameters in both scrotal and abdominal testes. Testicular T cell numbers were positively correlated with the number of testicular resident macrophages, which also were reduced by subcutaneous testosterone implants. The concentration of lymphocyte-inhibiting activity in the testicular interstitial fluid was reduced by 80% in short-term (1 month) and longer-term (3 months) cryptorchidism. These data indicate that the T cell population, and in particular the CD8(+) T cell subset, in the rat testis is functionally related to the resident macrophages or Leydig cells. On the other hand, testicular lymphocyte-inhibiting activity does not appear to be a determinant of the number of testicular T cells, and may not be a major factor in the prolonged survival of certain grafts in the abdominal testis.