Crude mitochondrial synaptosomal (P2) fractions were used to measure L- glutamate 1-decarboxylase (GAD) activity, and crude synaptic membranes were isolated from rat brains and used to determine gamma-aminobutyric acid (GABA) concentration and postsynaptic GABA receptor binding characteristics in rats with cobalt, copper, or glass implanted in right and left cerebral cortices. Copper was employed as a positive metal control because it elicits a morphological profile similar to that of cobalt but is non-epileptogenic. From tissue adjacent to the lesion, GAD activity was assessed by counting trapped 14CO2 liberated from [14C]glutamate and was reduced maximally to 25% of glass controls 7 days following cobalt insult, a period of peak seizure incidence. No reduction in GAD activity was observed 1 or 21 days after cobalt treatment or at any time period in copper-or glass-treated animals. A radioligand [3H]GABA receptor assay was utilized to determine GABA levels, postsynaptic receptor number (Bmax), and the affinity of the postsynaptic receptor for the ligand (KD) in tissue surrounding the lesion. GABA concentration was reduced maximally to 47% of glass controls 7 days following cobalt implantation. Scatchard plot analysis of tissue adjacent to the cobalt lesion revealed a significant increase in apparent receptor density (Bmax) to 200% of glass controls 7 days after bilateraL cobalt implantation (Bmax = 3.97 +/- 0.83 pmol/mg of protein, cobalt versus 1.36 +/- 0.17, glass control). Moreover, at 7 days, no change in kinetic parameters was noted after copper treatment. From days 7 to 21, the density (Bmax) of postsynaptic GABA receptors in cobalt-treated tissue appears to return slowly to glass control values. Results from the present study suggest that degeneration of the GABA pathway in the frontal cortex of the cobalt-epileptic rat occurs and, coupled with the increased Bmax, may represent a “denervation supersensitivity” phenomenon.