Evidence has accumulated showing that the alpha 1-adrenergic antagonist prazosin decreases nuclear estrogen binding in the hypothalamus of the guinea-pig. In this study we asked if prazosin treatment alters estrogen receptor (ER) protein content as reflected by changes in ER-immunoreactivity. The monoclonal rat antibody H222 directed against ER was used to detect ER-immunoreactive (ER-ir) cells in eight specific preoptic and hypothalamic brain regions of ovariectomized Hartley strain guinea-pigs treated with estradiol benzoate and 1.0 mg/kg prazosin or vehicle. Immunocytochemical parameters which provided optimum conditions for detection of even modest changes in ER-immunoreactivity were first established. Then, using these optimum conditions, we compared 1) the mean number of ER-ir profiles, 2) the mean density of ER-ir staining, and 3) the distribution of ER-ir staining density readings, between conditions within each of the eight brain regions. No differences in any of these measures were found between prazosin- and vehicle-treated females. We also compared the percentage of ER-ir nuclear profiles across the eight cell groups investigated in estradiol benzoate- and vehicle-treated females. The medial preoptic area had by far the highest percentage (48%) of ER-ir profiles (P < 0.05) compared to all seven other brain regions (23% to 32% ER-ir cells). Our data, showing that ER-immunoreactivity is not reduced (6h) after prazosin treatment, suggests that mechanisms other than alterations in ER protein should be considered when interpreting the effects of prazosin on the retention of estradiol by nuclear or cytosolic extracts of hypothalamic lysates.