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Improved glucose tolerance to sequential glucose loading (Staub-Traugott effect) is an important determinant of day-to-day glycemic exposure. Its mechanisms have not been clearly established. We recruited 17 healthy volunteers to receive two sequential oral glucose tolerance tests (OGTTs), at time 0 min and 180 min (Study I). The protocol was repeated on a separate day (Study II) except that plasma glucose was clamped at 8.3 mmol/1 between 60 and 180 min. [beta]-Cell function was analyzed by mathematical modeling of C-peptide concentrations. In a subgroup, glucose kinetics were measured by a triple-tracer technique (infusion of [6,6-[sup.2][H.sub.2]]glucose and labeling of the 2 glucose loads with [1-[sup.2]H]glucose and [U-[sup.13]C]glucose). In both Studies I and H, the plasma glucose response to the second OGTT equaled 84 [+ or -] 2% (P = 0.003) of the response to the first OGTT. Absolute insulin secretion was lower (37.8 [+ or -] 4.3 vs. 42.8 [+ or -] 5.1 nmol/[m.sup.2], P = 0.02), but glucose potentiation (i.e., higher secretion at the same glycemia) was stronger (1.08 [+ or -] 0.02- vs. 0.92 [+ or -] 0.02-fold, P = 0.006), the increment being higher in Study II (+36 [+ or -] 5%) than Study I (+19 [+ or -] 6%, P < 0.05). In pooled data, a higher glucose area during the first OGTT was associated with a higher potentiation during the second OGTT (rho=0.60, P = 0.002). Neither insulin clearance nor glucose clearance differed between loads, and appearance of glucose over 3 h totalled 60 [+ or -] 6 g for the first load and 52 [+ or -] 5 g for the second load (P = not significant). Fasting endogenous glucose production [ 13.3 [+ or -] 0.6 [micro]mol x [min.sup.-1] x kg fat-free mass [(FFM).sup.-1]] averaged 6.0 [+ or -] 3.8 [micro]mol x [min.sup.-1] x kg [FFM.sup.-1] between 0 and 180 min and 1.7 [+ or -] 2.6 between 180 and 360 min (P < 0.03). Glucose potentiation and stronger suppression of endogenous glucose release are the main mechanisms underlying the Staub-Traugott effect. glucose potentiation; glucose absorption; glucose tolerance