Morphological analysis of brains from rats receiving a convulsant dose of the muscarinic cholinergic agonist, pilocarpine hydrochloride (380 mg/kg), revealed a widespread damage to the forebrain as assessed by light microscopy 5-7 days after seizures. The substantia nigra, olfactory cortex, amygdala, hippocampus, septum, temporal cortex and thalamus underwent prominent morphological injury and cell loss. A concurrent assessment of the activity of L-glutamate decarboxylase (GAD), the gamma-aminobutyrate (GABA) synthesizing enzyme, demonstrated marked deficits in GAD activity in the brain regions undergoing morphological insult. Diazepam, 10 mg/kg, and scopolamine hydrochloride, 10 mg/kg, administered 30 min prior to the injection of pilocarpine, 380 mg/kg, prevented acute behavioral and electrographic, and long-term morphological and biochemical sequelae of seizures. These findings suggest that the muscarinic antagonist, scopolamine, and the anticonvulsant benzodiazepine, diazepam, may aid in preventing extensive brain damage related to pathological muscarinic cholinergic overactivity. The similarity of the topography of the damage and deficits in the GAD activity in brains of rats treated with pilocarpine indicates that GABAergic neurons are lost in the subregions of the brain preferentially sensitive to the convulsant action of pilocarpine.