To evaluate whether the gamma-aminobutyric acid (GABA)ergic system plays a role in the defective insulin secretion in human diabetes mellitus, 15 non-insulin-dependent diabetics with fasting hyperglycemia above 140 mg/dl were submitted to two consecutive i.v. glucose tolerance tests (IVGTT) (0.33 g/kg b.w.), in basal conditions and after pharmacologic activation of the GABA system with baclofen and sodium valproate. Baclofen, a synthetic analogue, was given to 8 diabetics in two divided doses of 10 mg each 8h and 1h before the post-treatment test; sodium valproate, a drug that increases endogenous GABA activity, was given orally (800 mg) 60 min before the performance of the post-treatment IVGTT. Neither treatment brought about significant changes in insulin, C-peptide, glucagon or growth hormone responses to i.v. glucose nor did they significantly change glucose disappearance rates. These results seem to indicate that GABA does not play a major role in the pathogenesis of defective insulin secretion in non-insulin-dependent diabetes mellitus.