Background.We have previously shown that cytotoxic T lymphocytes (CTL) with alloreactivity were induced when Wistar Furth (WF; RT1u) rats were immunized with allogeneic Brown Norway (BN; RT1n) cells. In contrast, when BN rats were immunized with WF cells, the allospecific response was confined to alloreactive natural killer (NK) cells, and no CTL activity was observed. In this study, the effect of cyclosporine (CsA) on the activation of alloreactive NK cells in vivo was analyzed.Methods.Distinct peritoneal effector cells from rats immunized with allogenic cells with or without concomitant CsA and/or interleukin (IL) 2 treatment were tested for specific cytolytic activity. Furthermore, the presumptive role of NK cells in rejection immunity was addressed in a cardiac graft model.Results.The results showed that doses of CsA that completely inhibited the activation of alloreactive CTL, only marginally affected the activation of alloreactive NK cells. We also showed that CsA treatment failed to prolong graft survival in BN recipients of WF hearts. Treatment of BN rats with CsA/IL-2 during immunization with allogeneic WF cells resulted in concomitant induction of alloreactive NK cells and alloreactive CTL.Conclusions.We have demonstrated that CsA failed to suppress the activation of alloreactive NK cells. Consequently, the cardiac graft survival in the donor-recipient combination known to activate alloreactive NK cells was not significantly prolonged by CsA treatment, emphasizing the involvement of NK cells as effectors in organ rejection. Furthermore, the parallel emergence of alloreactive NK cells and CTL only in the presence of CsA/IL-2 indicated that CsA interfered with alloreactive NK cell-associated suppression of CTL activated by allogeneic tissue.