Metformin was reported to increase plasma active glucagon-like peptide-1 (GLP-1) in humans. There are two possible mechanisms for this effect: (1) metformin inhibits dipeptidyl peptidase IV (DPPIV), an enzyme degrading GLP-1, and (2) metformin enhances GLP-1 secretion. To elucidate the mechanism(s), we examined (1) IC50 of metformin for DPPIV inhibition, (2) plasma active GLP-1 changes after oral biguanide (metformin, phenformin, and buformin) treatment in fasting DPPIV-deficient F344/DuCri rats, and (3) plasma intact GLP-1 excursions after oral administration of metformin and/or valine-pyrrolidide, a DPPIV inhibitor, in fasting DPPIV-positive F344/Jcl rats. Our in vitro assay showed that metformin at up to 30 mM has no inhibitory activity towards porcine or rat DPPIV. Metformin treatment (30, 100, and 306 mg/kg) increased plasma active GLP-1 levels dose-dependently in DPPIV-deficient F344/DuCrj rats (similar to1.6-fold at 3 and 5 It after administration of 300 mg/kg). This treatment had no effect on blood glucose levels. Similarly, phenformin and buformin (30 and 100 mg/kg) elevated plasma intact GLP-1 levels in F344/DuCrj rats. In DPPIV-positive F344/Jcl rats, coadministration of meiformin (300 mg/kg) and valine-pyrrolidide (30 mg/kg) resulted in elevation of plasma active GLP-1, but neither metformin nor valine-pyrrolidide treatment alone had any effect. These findings suggest that metformin has no direct inhibitory effect on DPPIV activity and that metformin and the other biguanides enhance GLP-1 secretion, without altering glucose metabolism. Combination therapy with metformin and a DPPIV inhibitor should be useful for the treatment of diabetes. (C) 2002 Elsevier Science (USA). All rights reserved.