Most of the criteria for identification of a neurotransmitter were satisfied for γ-aminobutyric acid (GABA) in the mammalian intestine. 1) GABA and its synthesizing enzyme, glutamic acid decarboxylase, and the neurons which specifically accumulate GABA were demonstrated to localize in Auerbach's plexus of the intestine. 2) GABA was demonstrated to be released from nerve terminals of the intestine when the nerve fibers were stimulated. 3) The application of GABA depolarized the neurons within Auerbach's plexus. 4) The actions of GABA were mimicked by muscimol on the GABAA receptor and by baclofen on the GABAB receptor. The GABAA antagonist is bicuculline, but no antagonist to GABAB is known at present. Thus, GABAergic neurons may be present in the enteric nervous system of the intestine. GABA and bicuculline changed the propulsive activity and the spontaneous motility of circular muscle, and the neuronal interactions, substance Pergic-GABAergic-postganglionic cholinergic neurons were found in the enteric nervous system, thereby suggesting that GABAergic neurons play a key role in the control of peristalsis.