Histological analyses have identified lymphocytes and macrophages as the predominant leukocyte populations that infiltrate organs undergoing chronic rejection. In order to define the time frame of this infiltration, we investigated the in vivo migration pattern of lymphocytes in a well-established rat model of chronic kidney allograft rejection. F344 kidneys were orthotopically transplanted into bilaterally nephrectomized Lewis rats. Recipients were treated with cyclosporin A (1.5 mg/kg/per day) for the first 10 days. After anti-CD18 or vehicle pretreatment, peripheral blood lymphocytes obtained from naive Lewis rats and labeled with 3H-uridine were injected into transplanted rats 12 and 16 weeks after transplantation. Organs were harvested 4, 8, and 12 h thereafter. After 12 weeks, proteinuria developed, accompanied by all signs of chronic rejection including glomerular sclerosis. Labeled lymphocytes rapidly infiltrated grafted kidneys 4 h after injection. Even more lymphocytes had accumulated in the grafts 12 h after injection. After 16 weeks, few lymphocytes had emigrated into the graft at 4 h, while infiltration was most pronounced by 12 h. Pretreatment with anti-CD18 inhibited the influx of lymphocytes. There was no difference between the patterns of lymphocytes derived from naive and transplanted rats. Our results emphasize the importance of endothelial cells in chronically rejecting kidneys for the control of leukocyte influx. Beta2-integrins may play a central role in determining the transendothelial migration during this process.