Cytoplasmic and nuclear accumulation of glycogen granules in the kidney cells of 72 male WBN/Kob rats with a long-term diabetic condition was studied histologically and by electron microscopy. The incidence and degree of the accumulation showed good correlation with the blood glucose concentration. In the kidneys, there was evidence of two types of lesion, cytoplasmic glycogen accumulation in the distal convoluted tubules and nuclear accumulation in the ascending thick segment of Henle's loops. Electron microscopically, the cytoplasmic glycogen accumulation was often associated with an increased number of lysosomal bodies containing lamellar bodies. Glycogen bodies, the halo of which was thought to be identical with that of nuclear bodies, were frequently observed in the nuclei containing the glycogen granules. These morphological and topographical differences between the two types of lesion were considered suggestive of different pathogenetic mechanisms for glycogen accumulation in the kidney cells.