The glycogen content of several types of supporting cells in the organ of Corti of the rat was demonstrated histochemically at the ultrastructural level using the periodic acid-thiocarbohydrazide-silver proteinate method. The development of the glycogen stores was assessed by means of a semiquantitative method in normal and congenitally-hypothyroid rats. At birth, all the cell types of the developing organ of Corti showed in their cytoplasms numerous glycogen particles. As development proceeded, the density of glycogen particles increased, reaching the highest value at the 8th postnatal day for the cells of the organ of Kölliker, inner pillar cell and outer pillar cell. On the other hand, the peak of maximum glycogen content in Deiters' cells was accomplished at the 15th postnatal day. From the day in which the maximum value was obtained onwards, the glycogen content in all the cell types fell and disappeared. Congenital hypothyroidism induced by propylthyouracil only affected the normal development of the density of glycogen particles in Deiters' cells, which didn't undergo the increase observed in the normal animals, remaining at values similar to those obtained at birth. This finding, together with previous similar results on the glycogen content of the inner ear's outer hair cells suggest that hypothyroidism selectively impairs the development of those organ of Corti's cells which mature on the second postnatal week, in the rat.