The Brazilian opossum (Monodelphis domestica) is a small, pouchless marsupial whose young are born in an immature, sexually undifferentiated state. Etgen and Fadem, and Handa and coworkers have biochemically detected and characterized estrogen receptors in the forebrain of the Brazilian opossum. In this study, we have examined the distribution of estrogen receptor-like immunoreactive (ER-LI) cells in the brains of gonadectomized male and female Brazilian opossums using Abbott H222 rat monoclonal estrogen receptor antibody (H222 is a gift of Abbott Labs). An indirect immunohistochemical procedure employing the Vectastain Elite system and a nickel-enhanced DAB chromogen was used. A large number of ER-LI cell nuclei were observed in the medial preoptic area, ventral septal nucleus, medial division of the bed nucleus of the stria terminalis, lateral part of the ventromedial hypothalamus, premammillary nucleus, arcuate nucleus, posterior amygdaloid nucleus, and the midbrain central grey. Lower numbers of ER-LI cell nuclei were observed in the intermediate subdivision of the lateral septal nucleus, and in the anterior, medial, and posterior cortical amygdaloid nuclei. The anatomical distribution of ER-LI in the Brazilian opossum brain is similar to that which has been reported for estrogen binding sites following biochemical analysis. Based on these findings, we believe specific regions of the Brazilian opossum brain may serve as substrata for the action of estrogen in the adult. In addition, these results are supportive of the use of this animal model to investigate the organizational effects of estrogen on the developing central nervous system.