In the hippocampus of female rats, steroid dependent changes in structure and function occur across the estrous cycle. Dendritic spine density is upregulated by estrogen and downregulated by progesterone. Progesterone that follows estrogen exposure rapidly decreases spine density in the CA1 region of the hippocampus and this action is blocked by the progestin receptor (PR) antagonist, RU486 (Woolley and McEwen, JCN 336: 293 – 306, 1993). Our previous studies have located immunoreactivity (-ir) for the alpha and the beta forms of the estrogen receptor in dendritic spines (Milner et al. JCN 429: 355 – 371, 2001; Milner et al., SFN, 2004). This study sought to describe the cellular and subcellular localization of PRs in the hippocampus. Coronal hippocampal sections from both proestrus and ovariectomized estrogen-replaced rats were immunolabeled with antibodies to PR and examined by light and electron microscopy. By light microscopy, PR-ir was undetectable in the hippocampal formation although PR-ir was presen...