The intracellular locations of the components of the heterooligomeric progesterone receptor (PR), heat-shock protein (hsp90), and the ligand-binding component were studied by immunoelectron microscopy in the chick oviduct, using immunogold double labeling and peroxidase techniques with monoclonal antibodies (MAb) against hsp90 (7D alpha and 4F3) and against PR (PR6 and PR13). PR was located in the nuclei of epithelial cells independently of the presence or absence of ligand. Cells with apically located nuclei were often PR negative. Ten minutes after progesterone administration no apparent change was seen in PR immunoreactivity, but chromatin underwent extensive rearrangement and PR was seen at the margins of the hetero- and euchromatin. The nucleoli did not contain PR. Hsp90 was located in the cytoplasm as aggregates, often inside small vesicles. In the apical part of the cell, hsp90 was located at the Golgi complex. The nuclei contained no detectable amounts of hsp90 except for that in the nucleoli. Ten minutes after progesterone administration the location or immunoreactivity of hsp90 did not alter. Thus, there seems to be a clear difference in the intracellular distribution of PR and hsp90. The epithelium also exhibited some cells with high levels of hsp90 and no or low levels of PR. These results raise the question of whether PR is associated with hsp90 in intact cells.