Androgenic steroids have been implicated in the development of sex differences in Xenopus laevis frogs. In order to determine when neurons first acquire the ability to concentrate androgen, we prepared autoradiograms of CNS in developing frogs following injection of tritiated dihydrotestosterone (DHT). X. laevis tadpoles and juveniles from stage 60 to 2 months post-metamorphosis (PM) were injected with [3H]DHT. Brain and spinal cord autoradiograms from these animals were examined for the presence of labelled cells. The pattern of [3H]DHT labelling in stage-64 tadpoles and in PM juveniles was similar but not identical to that seen in adults. Heavily labelled cells were seen in the motor nucleus of cranial nerves IX and X, medullary reticular formation, a presumed sensory nucleus of cranial nerve V, pretrigeminal nucleus of the dorsal tegmental area of the medulla, laminar nucleus of the torus semicircularis, anterior pituitary, ventral thalamus and anterior spinal cord. The vestibular sensory nucleus of cranial nerve VIII was the only area that concentrates DHT in adults but did not contain labelled cells in young animals. No [3H]DHT-labelled cells were found in stage-60 tadpoles. The onset of androgen concentrating capability in X. laevis CNS thus probably occurs between stages 60 and 64.