In the pilocarpine model of chronic limbic seizures, subpopulations of glutamic acid decarboxylase (GAD)-containing neurons within the hilus of the dentate gyrus and stratum oriens of the CA1 hippocampal region are vulnerable to seizure-induced damage. However, many gamma-aminobutyric acid (GABA) neurons remain in these and other regions of the hippocampal formation. To determine whether long-term changes occur in the main metabolic pathway responsible for GABA synthesis in remaining GABA neurons, the levels of mRNA and protein labeling for the two forms of GAD (GAD65 and GAD67) were studied in pilocarpine-treated animals that had developed spontaneous seizures. Qualitative and semiquantitative analyses of nonradioactive in situ hybridization experiments demonstrated marked increases in the relative amounts of GAD65 and GAD67 mRNAs in remaining hippocampal GABA neurons. In addition, immunohistochemical studies demonstrated parallel increases in the intensity of terminal labeling for both GAD65 and GAD67 isoforms throughout the hippocampal formation. These increases were most striking for GAD65, the isoform of GAD that is particularly abundant in axon terminals. These findings demonstrate that, in a neuronal network that is capable of generating seizures, both GAD65 and GAD67 are up-regulated at the gene and protein levels in the remaining GABA neurons of the hippocampal formation. This study provides further evidence for the complexity of changes in the GABA system in this model of temporal lobe epilepsy.