We investigated the time course of GFAP levels in the hippocampal formation during the first 24 h following soman intoxication in rats. GFAP mRNA expression was detected by in situ hybridization. Intense GFAP mRNA expression was present in the molecular layer of the dentate gyrus as early as 6 h after intoxication. This expression was comparatively lower in other dentate gyrus layers and hippocampal CA1, CA3 and CA4 areas and seemed to be related to excessive neuronal activity. Histopathological examination demonstrated that GFAP expression in dentate gyrus is not correlated with lesions. The high astrocytic reactivity in the molecular layer of the dentate gyrus is discussed in relation to the maintenance of the homeostasis of glutamate and of synaptic plasticity in this area during soman intoxication.