The presence of estrogen receptor cells in postnatal cerebral cortex and their topographical and numerical changes between birth and postweaning were examined. On postnatal days 0, 2, 8, 12, 18, and 25, six mice (three males and three females) were injected sc with 0.25 microgram/100 g BW 11 beta-methoxy-16 alpha-[125I]iodoestradiol [( 125I]MIE2). Two additional males on postnatal day 2 were each sc injected with 250 micrograms/100 g BW 17 beta-estradiol 1 h before radiolabeled ligand to establish the specificity of nuclear label. Two hours after the injection of [125I]MIE2 brains were frozen, and 4-microns sections were thaw-mounted and processed for autoradiography. Autoradiograms were exposed for 1-45 days, and cortical cells with nuclear uptake and retention of [125I]MIE2 were evaluated at the levels of the frontal pole, preoptic area, and central and posterior hypothalamus. At birth, cells with nuclear label were found predominantly in deep cortical layers. Between birth and postnatal day 2, the number of labeled cells increased in deep and intermediate laminae and first appeared in certain superficial regions. By day 8, labeled cells were concentrated in laminae II-VI of the cingulate/paracingulate and suprarhinal cortex. On day 12, labeling in laminae V and VI declined to a few cells, while a concentration of labeled cells remained in laminae II and III of the cingulate/paracingulate and suprarhinal regions. With subsequent development, an attenuation in labeling at all cortical levels was observed. By day 25, a small cluster of labeled cells remained in lamina II and III of the anterior cingulate, paracingulate, and suprarhinal regions, with additional labeled cells scattered throughout the remaining cortex. Competition with unlabeled 17 beta-estradiol reduced nuclear concentration of ligand in all cortical layers and demonstrated the specificity of [125I]MIE2 for the estrogen receptor. These results show the extensive presence of estrogen target cells in the early postnatal cortex and a profound change in topography and number of target cells during the postnatal period. These findings further suggest an important role for estrogen in the development of certain cortical neurons, possibly involving neuronal differentiation, cell positioning, and connectivity.