The author has investigated the ultrastructural alterations of renal cortex in the shock state which was induced by E.coli endotoxin. The cytochemical study using horseradish peroxidase as a tracer and Na+-K+ ATPase has been performed in an attempt of correlation between morphological changes and functional impairment. In the initial stage after endotoxin administration, vascular changes which might be justified as morphological evidences for microcirculatory derangement were observed. In four hours after administration, aggregates of platelets and fibrin in the glomerular-loop were noted. The proximal tubuli were also involved in various extent of pathological changes compared with the distal tubuli. The increased vascular permeability was substantiated by the peroxidase labeling-method, however, the decreased glomerular permeability was confirmed which may represent the decreased glomerular filtration capacity. The reabsorption of the tracer in the proximal tubuli was initially increased, however, in later, the reabsorption was marked decreased, and similar alteration of membrane-linked Na+-K+ ATPase was also existed in the distal tubuli. In these observations, the tubular reabsorption mechanism was temporarily increased as the rebound reaction to maintain the biological equilibrium, however, on account for persisting microcirculatory derangement, sever functional impairment may occur which eventually leads to irreversible dysfunction of the kidney.