LBNF1 cardiac allografts are rejected within 36 hr in Lewis (LEW) rats sensitized with Brown Norway (BN) skin grafts (acute rejection = 7.5 days). We analyzed the effects of intrathymic versus intravenous alloantigen challenge upon graft survival in this well-defined accelerated rejection model. Intrathymic injection of LBNF1 spleen cells (2 x 10(7)) at the time of skin transplantation (day -7) abrogated < 36 hr rejection, and prolonged the survival of cardiac allografts to about 11 days. This striking effect did not require concomitant immunosuppression, and was donor specific, as the transfer of syngeneic (LEW) or third-party (Wistar-Furth) splenocytes was ineffective. X-irradiation of donor spleen cells before inoculation restored accelerated rejection, whereas thymectomy at day -6 or 0 (the day of heart transplant) significantly shortened graft survival. In contrast, although intravenous administration of the same number of donor cells into sensitized recipients prolonged cardiac allograft survival to about 9 days, the effect was x-irradiation resistant and was never influenced by thymectomy. Radioactive tracer studies have revealed distinct trafficking patterns for the transferred cells, with those given intrathymically retained mostly in the thymus, and sequestered into host spleen and lymph nodes. Instead, intravenously injected splenocytes did not accumulate in the thymus, but were eventually trapped in the liver. Moreover, intrathymic immunomodulation has switched off cellular, rather than humoral, events at the graft site, and markedly depressed cell proliferative responses in host lymphoid organs, as analyzed by immunohistology and mixed lymphocyte response (MLR)* assay, respectively. In contrast, intravenous therapy did not have any significant effect on early intragraft cellular infiltration, including considerable neutrophil infiltration, and did not affect lymph node cell proliferation in vitro. These data document the importance of the thymus as a potential target organ for modulation of alloreactivity in vivo, and reinforce the role of distinct "central" and "peripheral" host immune mechanisms contributing to immunological unresponsiveness following organ transplantation.