The distribution of estrogen target neurons is assessed in the forebrain of larval sea lamprey, Petromyzon marinus, by the use of thaw-mount autoradiography. Following the injection of [3H]estradiol-17 beta, radioactively labeled neurons are found in the ventral telencephalon and in the ventral and dorsal diencephalon, including preoptic, central hypothalamic, and thalamic regions. In the pallium no labeled cells exist. Pituitary glands, obtained from two of the animals, contain no labeled cells. The topographical distribution of estrogen target neurons in larval lamprey is similar to that of adult animals. The number of target neurons in larval lamprey, however, is lower than in the adult. The presence of target cells indicates that gonadal steroids act on the brain at this early stage of development. The lack of concentration of estrogen in pituitary cells suggests an absence of feedback regulation of estrogen at the pituitary level.