Effect of acute inhalation exposure of toluene or halothane anaesthesia on extracellular levels of gamma-aminobutyric acid (GABA) was monitored within the cerebellum of rats by microdialysis. GABA increased during and after exposure to toluene (2000 p.p.m., 2 hr) in contrast, halothane had no noticeable effect on GABA levels. When tetrodotoxin was added to the perfusion medium basal concentrations of GABA decreased to about 74% of control concentrations. Extracellular GABA levels did not increase during exposure to toluene when tetrodotoxin was added to the perfusion medium. The results indicate that toluene increase GABA within the cerebellum by sodium dependent mechanisms, possibly by modulating the neuronal input from the mossy fibers to the cerebellar cortex.