We mapped the distribution of estrogen receptor-containing cells in the forebrain of the adult female guinea pig. Cellular estrogen receptor content was detected using monoclonal antibody H222, directed against the estrogen receptor, and the avidin-biotin method with nickel-intensified diaminobenzidine as the chromagen. A complete set of deletion, titration, and adsorption controls established the specificity of the staining. The most dense collections of estrogen receptor-immunoreactive cells were found in medial preoptic, medial hypothalamic, and limbic nuclei (amygdala, bed nucleus of the stria terminalis, lateral septum). Numerous estrogen receptor-immunoreactive cells were also found in additional, specific subregions of the remainder of the preoptic area, hypothalamus, and limbic system, and also in the midbrain (central gray). Elsewhere, estrogen receptor-immunoreactive cells were present in smaller numbers or were absent. This map confirms and extends previous maps based on estrogen binding. The majority of estrogen receptor-immunoreactive cells are found in areas known to be involved in some aspect of reproduction. In addition, many estrogen receptor-immunoreactive cells are found in areas not typically considered to have a primary role in reproductive behavior or neuroendocrine function.