The action of inhibitory amino acid transmitters GABA, glycine, beta-alanine and taurine has been studied on the membrane potential of cultured astrocytes and on the extracellular K+-concentration ([K+]0) using K+-sensitive microelectrodes. All four amino acids caused a depolarization of glial cells and an increase of [K+]0. The effects produced by GABA were usually more pronounced than those caused by the other amino acids. Simultaneous recordings of the action of GABA and glycine on the glial membrane potential and on [K+]0 usually revealed a good correlation in time course, but often there were differences between the amplitudes of glial depolarizations and the values calculated from the [K+]0 increase. 4-Aminopyridine, which blocks K+-conductance of excitable membranes, reversibly abolished both the glial depolarization and the [K+]0 increased produced by GABA and glycine. From these results it is concluded that unlike neurones, glial cells do not have receptors for these amino acid transmitters and that their action on glial cells is caused by the efflux of K+ from activated neurones.