The ultrastructural localization of steroid hormone receptors has been made possible by the development of immunocytochemical procedures using monoclonal antibodies. Estrogen receptor-immunoreactivity (ER-IR) in the brain is present most abundantly in neuronal nuclei when observed with light microscopy. However, we have also observed ER-IR in the perikarya and cytoplasmic processes of neurons. To determine the organelles with which the cytoplasmic ER-IR is associated, we developed a technique for ultrastructural visualization of ER-IR. Ovariectomized guinea pigs were perfused, brains vibratome-sectioned, and estrogen receptors immunostained by either an immunoperoxidase-diaminobenzidine technique or by an immunogold-streptavidin procedure, each followed by silver intensification. Electron microscopic analysis confirmed distribution of ER-IR throughout cell nuclei, but ER-IR was also observed in proximal and distal dendrites and rough endoplasmic reticulum. Most surprisingly, however, ER-IR was found in many axon terminals containing predominantly round, and in some cases, flattened clear synaptic vesicles. Parallel experiments examining the distribution of progestin receptors confirmed the localization at the same subcellular sites as for estrogen receptors. The results of this experiment corroborate our earlier findings of extranuclear steroid receptor-immunoreactivity in the brain, and they suggest potential nongenomic sites of action for estradiol and progesterone in dendrites and axon terminals.