The high concentrations of gamma aminobutyric acid (GABA) in the pancreatic islets and the neurotransmitter role played by this amino acid in the central nervous system, make it plausible that GABA also intervenes in the control of endocrine pancreatic function. In 12 normal subjects, a single oral dose of 5 or 10 g GABA, as compared to placebo, caused a significant (p less than 0.01) and dose-dependent (p less than 0.01) increase of plasma levels of immunoreactive insulin, C peptide and glucagon, without affecting plasma glucose concentration. By contrast, in 15 additional subjects, a single oral dose of 5 mg muscimol, a specific GABA receptor agonist, did not consistently influence the above parameters. Although the lack of effects of muscimol might indicate that the action of GABA is not mediated through specific receptors, the results with GABA suggest that this amino acid plays a specific role in the regulation of endocrine pancreatic function.