Cytochrome oxidase, succinic dehydrogenase, and succinate oxidation were studied in the anterior, middle, and posterior hypothalamus during the sexual cycle in normal female rats as well as in rats rendered persistent-estrous by androgen treatment soon after birth. Similar values for succinate oxidation and succinic-dehydrogenase activity were found in the anterior hypothalamus of diestrous and androgenizedrats, and these values were significantly lower than those found in estrous rats. On the other hand, both biochemical parameters in the posterior hypothalamic area of estrous rats were similar to those observed in androgenized rats; such values were higher than those obtained in diestrous animals. No differences between the three groups of rats were apparent in the succinate oxidation and succinic-dehydrogenase activity of the middle hypothalamus. No significant changes were found in the cytochrome oxidase activity in the different hypothalamic areas of any of the groups of rats studied.