Despite the many effects of estrogen in the hippocampus, there has been little evidence that hippocampal principal cells express nuclear estrogen receptors. In the hippocampus, the α form of the nuclear estrogen receptor (ERα) has been localized to sparsely distributed cells with the morphological characteristics of inhibitory interneurons. Because inhibitory neurons may be involved in the effects of estrogen on hippocampal principal cells, quantitative description of ERα expression in γ-aminobutyric acid (GABA)ergic (inhibitory) and non-GABAergic cells of the hippocampus is a key step in understanding the mechanism(s) of estrogen action on hippocampal circuitry. We used single and double-label immunohistochemistry for ERα and glutamic acid decarboxylase (GAD; a marker of GABAergic neurons) to determine the numbers and distributions of hippocampal GABAergic and non-GABAergic neurons that express ERα in the adult female rat. We found many more ERα-expressing cells in the hippocampus than any previous study and observed distinct dorsal vs. ventral differences in hippocampal ERα expression. In the dorsal hippocampus, most ERα-positive cells were also GAD positive; however, ERα was expressed in only a subset of GAD-positive cells. Double-labeled cells were concentrated at the border between str. radiatum and str. lacunosum-moleculare. In the ventral hippocampus, we found a very high number of ERα-positive cells, the majority of which were not immunoreactive for GAD and are likely to be pyramidal cells. These findings suggest that ERα can mediate the effects of estrogen primarily in GABAergic neurons in the dorsal hippocampus and in both GABAergic and non-GABAergic neurons in the ventral hippocampus. J. Comp. Neurol. 440:144–155, 2001. © 2001 Wiley-Liss, Inc.