Exocytosis of the hormone, glucagon-like peptide-1 (GLP-1), by the intestinal L-cell is essential for the incretin effect after nutrient ingestion, and is critical for the actions of dipeptidylpeptidase IV inhibitors that enhance GLP-1 levels in patients with type 2 diabetes. 2-Photon microscopy revealed that exocytosis of GLP-1 is biphasic, with a 1(st) peak at 1-6min and a 2(nd) peak at 7-12min after stimulation with forskolin. Approximately 75% of the exocytotic events were represented by compound granule fusion, and the remainder were accounted for by full fusion of single granules, under basal and stimulated conditions. The core SNARE protein, syntaxin-1a (syn1a), was expressed by murine ileal L-cells. At the single L-cell level, 1(st) phase forskolin-induced exocytosis was reduced to basal (p<0.05) and 2(nd) phase exocytosis was abolished (p<0.05) by syn1a knockout. L-cells from intestinal-epithelial syn1a-deficient mice demonstrated a 63% reduction in forskolin-induced GLP-1 release in vitro (p<0.001), and a 23% reduction in oral glucose-stimulated GLP-1 secretion (p<0.05) in association with impairments in glucose-stimulated insulin release (by 60%, p<0.01) and glucose tolerance (by 20%, p<0.01). Our findings therefore identify an exquisite mechanism of metered secretory output that precisely regulates release of the incretin hormone, GLP-1 and, hence, insulin secretion following a meal.