Recent studies have indicated that hippocampal GABAergic neurons in both the dentate gyrus and Ammon's horn contain immunoreactivity for the calcium-binding protein parvalbumin (PARV). Although the distribution of PARV-positive neurons has been previously described, detailed quantitative electron microscopic studies of the PARV-positive axon terminals in the hippocampal formation are lacking. In the present study, immunocytochemical methods were used to localize PARV-positive neurons and axon terminals to determine their similarity to GABAergic neurons. The PARV-positive cells and axon terminals are associated closely with the pyramidal and granule cell layers. In agreement with previous studies, the morphology of PARV-positive neurons is similar to that of GABAergic cells, including the basket cells of both the dentate gyrus and Ammon's horn. The PARV-positive axon terminals form exclusively symmetric synapses with somata, dendrites, dendritic spines, and axon initial segments. However, these terminals represent only a portion of the total number of terminals that form symmetric synapses. Quantitative results indicate that only 32-38% of the total number of terminals forming symmetric axosomatic synapses with principal cells of the dentate gyrus and Ammon's horn are PARV positive. Together with previous findings from light microscopic double-labeling studies, these data indicate that the PARV-positive terminals arise from a subpopulation of GABAergic hippocampal neurons. Finally, it is important to note that the terminal plexus of PARV-positive hippocampal axons overlaps at all postsynaptic sites with a plexus of PARV-negative axons.