Tissues of Japanese quails were subjected to electron microscopical study. The birds had received 0–8 ppm methylmercury added to their diets for 6 weeks. The ultrastructural changes in the littoral cells of the liver were swollen mitochondria, with dilatation of the endoplasmic reticulum, loss of ribosomes from the rough endoplasmic reticulum and an increase in the number of vesicles in the cytoplasm. The nuclear membrane in some littoral cells became dilated, and the nuclear chromatin was condensed. The physiological effect of the destruction of the littoral cells is discussed.