Conversion of estradiol and estrone to catechol estrogens by rat hypothalamic tissue but not by the cerebral cortex was demonstrated from the incubation of these tissues with estradiol-2-3H and estrone-2-3H and monitoring the incorporation of tritium into water. Direct evidence for this transformation was obtained by isolating the labelled phenazine derivative of 2-hydroxyestrone after hypothalamic incubation with estrone-4-14C.