The intraventricular injection of kainic acid (KA) in rats produces a loss of dentate hilar neurons and hippocampal CA3 pyramidal cells, and renders the dentate granule cells and the CA1 pyramidal cells hyperexcitable. We have used immunocytochemical detection of glutamic acid decarboxylase (GAD), a marker of gamma-aminobutyric acid (GABA) cells, as well as stereological cell counting techniques, to determine whether inhibitory cell loss was present 2 weeks after KA treatment. In area CA1, we found that the density of GAD-positive cells was reduced by KA, but only in stratum oriens and the alveus. Counts of Nissl-stained neurons were also significantly reduced in this layer. These results demonstrate a loss of GABA cells in the basal dendritic layer of the CA1 region, which may underlie the hyperexcitability of CA1 pyramidal cells following KA treatment.