The inhibitory responses to glycine, taurine, beta-alanine, 2-amino-5-phosphonovaleric acid (APV) were investigated in freshly isolated neurones from medulla oblongata and hippocampus of rat using concentration clamp, voltage clamp and intracellular perfusion techniques. All substances, when applied in a steplike manner, induced a rapid increase in the chloride permeability of the cellular membrane which slowly desensitized. The responses to all substances were blocked by external strychnine. Cross-desensitization experiments demonstrated that all substances tested activate the same system of membrane receptors.