The distribution of the histochemical protein tracer, horseradish peroxidase, was studied in proximal tubules of rats with Heymann nephritis. Peroxidase reabsorption was substantially reduced in stage 2 of Heymann nephritis, a period during which the brush border of proximal tubules is severely damaged by specific antibodies. Impairment of the reabsorption function could not be attributed either to proteinuria or disturbances of proximal tubule metabolism and appeared to result from loss of microvilli. Recovery of brush border membrane morphology in stage 4 of Heymann nephritis was not accompanied by recovery of the normal capacity to reabsorb peroxidase. Functional deficits resulting from immunologic injury to proximal tubules in Heymann nephritis may persist despite waning of the anti-brush border antibody response and regeneration of the brush border of proximal tubule cells.