The Fas system-based rejection mechanism has not been studied well in terms of cytotoxic T cell activity in graft rejection. We investigated the Fas and Fas ligand level in renal grafts with acute and chronic rejection in a rat model using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR). Fas ligand in renal allografts was detected as early as 1 day after transplantation in an acute rejection model. It was highly expressed at day 4 and began to decline at day 6 after transplantation. In contrast, Fas ligand in normal kidneys was almost undetectable. Fas ligand in isografts was increased, but the expression level was much lower than in allografts. Interestingly, when Fas ligand expression began to decline in renal allografts, it increased in the spleens of recipients. Fas ligand expression in chronically rejecting allografts was slightly increased, but it was stronger than in isografts. In contrast to Fas ligand gene expression, Fas was constitutively expressed in isografts, allografts, and normal kidneys. However, the Fas level in renal allografts was higher than in normal kidneys. Our data demonstrated that the Fas system might play an important role in acute and chronic rejection by causing apoptosis, and the spleen may eliminate the lymphocytes strongly expressing Fas ligand after completion of the acute rejection.