GABA, best known as a neurotransmitter in the central nervous system, is also present in various peripheral tissues including the gastrointestinal tract, where there is strong evidence that GABA acts as a transmitter in some intrinsic myenteric neurones. Several studies indicate that the gastric mucosa is one of the sites of action of GABA in the gut. Highly specific anti-GABA antibodies have been used to detect endogenous GABA in the mucosa of the rat gastrointestinal tract, and 3H-GABA uptake followed by autoradiography has been used to localise cells with uptake sites for exogenous GABA. It was found that although GABA immunoreactive nerve fibres are essentially absent from this site, some mucosal cells are strongly GABA-immunoreactive. These cells are common in the pyloric stomach and upper part of the small intestine. The autoradiographic experiments provide evidence that these cells also possess high-affinity GABA uptake sites. These observations raise the possibility that in the gastrointestinal tract GABA acts as a gut hormone in a subpopulation of mucosal endocrine cells, in addition to its role as an enteric neurotransmitter.