With an aim to investigate the relative sensitivity of various renal structures to allograft rejection, we analyzed the histochemical reaction intensity of seven enzymes prominently displayed in various rat kidney components, and correlated the expression of these enzymes both to the degree of intra-graft inflammation and to the expression of class II MHC antigens in graft capillary endothelial cells. Syngeneic transplants and normal renal tissue were used as controls. At the peak of inflammation, on the fifth day after transplantation, adenosine triphosphatase activity of vascular endothelial cells was strongly reduced in the peritubular capillary endothelium of the allograft, moderately in the glomerular endothelium but very little in the endothelium of arteries and veins. Lactate dehydrogenase, succinate dehydrogenase, isocitrate dehydrogenase, alkaline phosphatase, acid phosphatase and glucose-6-phosphatase activities were moderately reduced in the proximal tubular cells of the allograft and even less in the distal tubular cells. The results suggest that the prime target of the host immune attack is the intertubular capillary endothelium, whereas the distal tubular cells are relatively insensitive to immune injury.