The development of peroxisomes, lysosomes and endocytic vacuoles in regenerating cells of the pars recta (P3 segment) of proximal tubules, in rats given a single interperitoneal injection of d-serine (80 mg/100 g.b.wt), was studied by light and electron microscopy using cytochemical methods. Rapid proliferation of cells occurred between 2 and 5 days after d-serine induced tubular necrosis; by day 6 almost all injured tubules were re-epithelialized with flat or low cuboidal cells. Peroxisomes and lysosomes were not observed during the period of rapid cell multiplication i.e., between 2 and 6 days after d-serine injection. Restitution of mitochondrial population preceded the development of peroxisomes in the newly regenerated cells of P3 tubules. Maximum development of peroxisomes occurred between 9 and 14 days after d-serine injection. The formation of peroxisomes appeared to correlate closely with the differentiation of apical endocytic vacuoles and the brush border. Lysosomes in the regenerated cells of P3 tubules were the last to develop.