Background: The incretin, glucagon-like peptide-1 (GLP-1), which is used to treat diabetes mellitus, delays gastric emptying by inhibiting vagal activity. GLP-1 also increases fasting and postprandial gastric volume in humans. Based on animal studies, we hypothesized that nitric oxide (NO) mediates the effects of GLP-1 on gastric volumes. Aims: To assess the effects of nitrergic blockade on GLP-1 induced gastric accommodation in humans. Methods: In this double-blind study, 31 healthy volunteers were randomized to placebo (i.e., saline), GLP-1, the nitric oxide synthase inhibitor L-NMMA (4 mg/kg/hr) alone or with GLP-1. Thereafter, 16 additional subjects were randomized to GLP-1 alone or together with a higher dose of L-NMMA (10 mg/kg bolus plus 8 mg/kg/h infusion). Gastric volumes (fasting pre- and post-drug, postprandial post-drug) were measured by ^99mTc-single photon emission computed tomography (SPECT) imaging. Results: GLP-1 increased (p = 0.04) fasting gastric volume by 83 ± 16 ml (versus 17 ± 11 ml for placebo) and augmented (p 0.01) postprandial accommodation by 688 ± 165 ml (versus 542 ± 29 ml for placebo). L-NMMA (low dose) alone did not affect fasting or postprandial gastric volume. L-NMMA (low dose) did not attenuate the effect of GLP-1 on gastric volumes. In contrast, L-NMMA (high dose) did not affect fasting volume but blunted GLP-1-mediated postprandial accommodation (postprandial change = 494 ± 37 ml, p 0.01 vs GLP-1 alone). Conclusions: These data are consistent with the hypothesis that NO partly mediates the effects of GLP-1 on postprandial but not fasting gastric volumes in humans.