The uptake of gamma-aminobutyric acid (GABA) by glial cells was decreased when 4,5,6,7,-tetrahydroisoxazolo-(4,5-C)-pyridin-3-ol (THPO) was applied in the thalamus of freely moving cats by in vivo microdialysis. A marked reduction in duration of wakefulness and in number of awakenings was obtained during THPO treatment. THPO did not change the ratio of slow-wave-sleep and paradoxical sleep but only increased the total sleep time. The present data suggest a possible regulatory role of the glial-neuronal interaction in the modification of the sleep-waking cycle.