The fine structure of rat liver cells was examined after 10 days of daily intraperitoneal administration of Metopiron. The hepatic cells presented hypertrophy of the smooth endoplasmic reticulum and an almost complete glycogen depletion. The relative volume of the smooth endoplasmic reticulum calculated by the stereological methods of Weibel and associates ( 45, 46) was shown to be significantly higher than that found in normal controls. Hepatic slices incubated in medium with Metopiron showed no ultrastructural alterations. The effect of Metopiron on the liver is discussed and related to an increase in the production of non 11- β-hydroxylated compounds by the adrenal.