The photoreceptor outer segments and retinal pigment epithelium of vitamin E deficient rats were examined by electron microscope using electron histochemical techniques. In rats fed a vitamin E deficient diet for more than 6 months starting 20 days after birth, the photoreceptor outer segment showed vesiculation of the discs near the apical portion of the retinal pigment epithelium. In the retinal pigment epithelium, secondary lysosomes were increased in number. Reaction products of acid phosphatase activity were located in secondary lysosomes in the retinal pigment epithelium and on the disc membranes of the photoreceptor outer segment. These alterations became more prominent with the duration of vitamin E deficiency. It was assumed that the disc degeneration of the photoreceptor outer segment might be due to enhanced activity of lysosomal enzymes in the retinal pigment epithelium and the effect of released lysomal enzymes on the photoreceptor outer segment.