Events in the early post-transplant period have been correlated with increased renal allograft loss. Immunologic reactions and ischemic injury have been implicated in this process. While the immunologic aspects of allograft injury have been studied extensively, ischemic effects remain less well understood. To study the effects of ischemia in rats with different genetic backgrounds without the introduction of an alloimmune response, a clamp was placed on the vascular pedicle of the left kidney for 60 min. The short-term effects (1 wk) of ischemia were studied in groups of PVG (RT1c), LEW (RT1), DA (RT1a) and WR (RT1u) rats, Immunoperoxidase staining demonstrated limited infiltration of monocytes, macrophages, and T-cells accompanied by upregulation of low levels of MHC class II antigens on tubular epithelial cells, peritubular capillaries, and interstitial cells in kidneys of PVG and WF rats. Kidneys of LEW and DA rats had greater influxes of monocytes, macrophages, and T cells in addition to higher amounts of MHC class II antigens upregulation on tubular epithelium and interstitial cells. The long-term effects of ischemia were studied in kidneys of WF rats. These kidneys had a progressive increase in infiltrating T cells, monocytes, macrophages and MHC class II expression on the tubular epithelium and the interstitial cells at 14, 30, and 90 d after the ischemic insult. The differences in MHC class II expression between ischemic kidneys of PVG and LEW rats were not associated with differences in production of mRNA for IL-2, IFN-gamma, and TNF-alpha. In summary, transient renal ischemia in the absence of an allogeneic immune response triggers a progression of inflammatory responses, including leukocyte infiltration, cytokine production and MHC class II antigen upregulation which appears to be strain-dependent.