Results of electron microscopic examinations of the membraneous cochlea of 6 untreated rats and 36 rats fed with keltane (chloroorganic pesticide) at a 1/20 LD50 dose are presented. Biosamples were withdrawn 30, 60 and 120 days after onset of treatment. In the spiral limb (SL) area, light or dark cells, fibroblasts, fibrillar structures, and blood capillaries were discerned. Light cells included few organelles. Dark cells contained secreting cells located near blood capillaries. Internal spiral notch (ISN) cells had osmiophobic cytoplasm, single organelles, and many microvilli. At an early experimental stage (day 30) light cells, fibroblasts, and blood capillaries proved most sensitive to keltane. They displayed mitochondrial swelling, Golgi organ hyperplasia, intercellular space enlargement, perivascular edema, thickening of the capillary endothelium, and basal membrane swelling. At day 60 edema and ultrastructural changes diminished and dark cells showed signs of enhanced activity. At a late stage (day 120) ultrastructural changes occurred in all SL and ISN organelles. The cytoplasm of some cells showed destructive changes while that of others displayed reparative changes. Ultrastructural changes in the SL and ISN produced a direct effect on toxic lesions of receptor elements of the spiral organ.