Pyroglutamic acid (1-PCA), a cyclic derivative of glutamic acid, was administered i.p. (7.7 mmol/kg) or intracerebroventricularly (25 - 50 mumol) to freely moving guinea-pigs, provided with semi-permanently implanted epidural cups. The effect of this compound on cortical Acetylcholine (ACh) and GABA outflow, as well as on gross behaviour and electrocorticogram (E.Co.G.) was investigated. 1-PCA increased the release of ACh and GABA from the cortical surface, did not change their cortical content, decreased the spontaneous motor activity and synchronized the E.Co.G.. These results suggest that 1-PCA increases GABA release, possibly by changing amino acid transport through the biological barrier or by acting as antagonist on the receptors for glutamic acid. In turn, the activation of the GABA system increases, as previously demonstrated, the cortical ACh release and causes mild sedation and E.Co.G. synchronization.