The numerical density of neurons in the CA1 region of the rat dorsal hippocampus has been estimated by a stereological method, the disector, using pairs of video images of toluidine blue-stained, plastic-embedded, 0.5-microns-thick sections, 3 microns distant from each other. The chemical properties of those disector-counted cells were further analyzed by postembedding immunocytochemical methods on adjacent, semithin sections using antibodies against gamma-aminobutyric acid (GABA) and a specific calcium-binding protein, parvalbumin (PV). The density of neurons in the CA1 region was 35.2 x 10(3)/mm3; numerical densities in the stratum oriens (SO), stratum pyramidale (SP), and strata radiatum-lacunosum-moleculare (SRLM) were 11.3 x 10(3)/mm3, 272.4 x 10(3)/mm3, and 1.9 x 10(3)/mm3, respectively. The numerical densities of GABA-like immunoreactive (GABA-LIR) and PV-immunoreactive (PV-IR) neurons were 2.1 x 10(3)/mm3 and 1.1 x 10(3)/mm3, respectively, which were 5.8% and 3.2% of all neurons, respectively. In the CA1 region only about 60% of PV-positive neurons were GABA-LIR. However, taking the previous observation into consideration that almost all hippocampal PV-positive neurons were immunoreactive for the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD), neurons that were immunoreactive to either GABA or PV or both (GABA+ and/or PV+ neurons) were regarded as a better representative of GABAergic neurons in this region; thus, the numerical density of these GABA+ and/or PV+ neurons was 2.5 x 10(3)/mm3 and they were 7.0% of all neurons in the CA1 region. Lamellar analysis showed that the numerical densities of GABA+ and/or PV+, GABA-LIR, and PV-IR neurons were highest in the SP, where they were 8.2 x 10(3)/mm3, 6.2 x 10(3)/mm3, and 5.4 x 10(3)/mm3, respectively. The results of the present study indicate that the proportions of GABAergic neurons and a subpopulation of them, PV-containing GABAergic neurons, to other presumable non-GABAergic neurons are far smaller in the CA1 region of the hippocampus than in several neocortical regions previously reported.