We have studied the distribution of gamma-aminobutyric acid (GABA) neurons, axons, and synapses in the rat and monkey hippocampal formation by using glutamate decarboxylase (GAD) immunocytochemistry together with Nissl stains, electron microscopy, and double-labeled retrograde transport of horseradish peroxidase. The numbers of GAD-containing (putative GABA) neurons and their percentages compared to all Nissl-stained neurons were calculated throughout all the various fields and strata of the mammalian hippocampus. Although their numbers are greatest in the polymorph region of the fascia dentata (FD) and in the principal cell layers stratum pyramidale (SP) and stratum granulosum (SG), GAD immunoreactive (GAD-IR) cells are numerous in other strata that contain mostly dendrites and scattered cells. These GAD-IR (putative GABA) neurons in dendritic regions may be involved in feedforward dendritic inhibition or may directly inhibit nearby neurons. We used a postmortem delay technique, which resulted in apparent diffusion of GAD into dendrites and axons and allowed better visualization of the extensive dendritic domain of GAD-IR neurons. Computerized image analysis of GAD-IR puncta indicated that putative GABA terminals were numerous on apical and basilar dendrites of all pyramidal cells but unexpectedly highest in the monkey presubiculum. In the rat, GAD-IR neurons projected axons ipsilaterally from every region to the fascia dentata and CA1; however, commissural GAD-IR axons to the fascia dentata arose from GAD-IR neurons in only the contralateral fascia dentata and subiculum. Electron microscopy of GAD-stained hippocampus identified GAD-IR neurons with non-GAD-IR (possibly excitatory) synapses and GAD-IR terminals on somata and dendrites, 80% being the symmetric type and 20% the asymmetric type. In contrast, non-GAD-IR terminals were asymmetric 80% of the time.