Fos immunoreactivity was observed in astrocytes identified by immunoreactivity to glial fibrillary acidic protein, 1.5 h following intravenous pentobarbitone anesthesia (20-30 mg/kg). Fos positive astrocytes were seen only in the hilus of the hippocampus. Pentobarbitone administered after picrotoxin but without seizures resulted in an increase in both the intensity of Fos labeling and areal distribution of Fos-positive hilar astrocytes. Pentobarbitone administered after single picrotoxin-induced seizures resulted in an increased number of Fos immunoreactive astrocytes in the hilus and other hippocampal regions and their presence in the upper layers of Fr1, Fr2, cingulate, visual, and perirhinal cortex. Our observations that Fos is expressed in astrocytes following pentobarbitone, provides support for the current view that glial cell processes can be activated by pentobarbitone stimulation of GABA, receptor-chloride channels.