Recent studies suggest that cytotoxic T-lymphocytes expressing p38 mitogen-activated protein kinase (p38MAP kinase) contribute to allograft rejection in clinical heart transplantation. Interleukin-2 (IL-2), a potent T cell mitogen, activates the p38MAP kinase pathway, resulting in phosphorylation of target transcription factors. In this study we investigated the expression of activated p38MAP kinase in intragraft cell infiltrates following rat heterotopic small bowel transplantation and examined the effects of the immunosuppressant FK506 on intragraft expression of activated p38MAP kinase and allograft rejection. Allografts receiving FK506 (0.5 mg/kg per day i. m.) for 7 days as primary anti-rejection therapy had a significant reduction in histopathological evidence of allograft rejection on Day 7, compared to allograft controls. In addition, Western blotting analysis of intragraft cell infiltrates showed a reduction in the expression of activated p38MAP kinase in allografts treated with FK506. We conclude that intragraft cell infiltrate expression of activated p38MAP kinase is an important marker of acute rejection in this animal model of small bowel transplantation, and that FK506 is an effective immunosuppressant, in this situation, that may act in part by preventing the activation of p38MAP kinase.