After intravenous administration of (5 x 2.3 kBq and 5 x 9.25 kBq) plutonium citrate in adult male Sprague Dawley rats, their kidneys are withdrawn and prepared for observation under a transmission electron microscope. Seven days after the first injections, deep cellular alterations are observed in the proximal convoluted tubules. These alterations are mainly mitochondrial. The affected mitochondria are of swollen aspect and have their cristae partially or completely destroyed. Nevertheless within the same tubule we observe non altered cells directly in contact with deeply altered cells. In all the cases the lysosomes of the altered cells appear to be perfectly normal. The cell nuclei are mostly unaltered but a few cases of nuclear fragmentation exist. We also notice some architectural modifications in the brush border and in the betacytomembranes of the proximal convoluted tubule. Equally important mitochondrial alterations are also noticed in the different varieties of glomerular cells. We observe no other glomerular alterations. The major subcellular alterations in the proximal convoluted tubules and in the different varieties of glomerular cells deeply contrast with the distal convoluted tubules which are found to be totally unaltered. These mitochondrial alterations may be due to the alpha particle disintegration of plutonium which may either directly react with the mitochondria or, through the products of radiolysis of water react with the mitochondria respiration process. However the direct chemotoxicity of plutonium cannot be neglected.