We have separated the effect of insulin on glucose distribution/transport, glucose disposal, and endogenous production (EGP) during an intravenous glucose tolerance test (IVGTT) using a dual tracer dilution methodology. Six healthy lean male subjects (age 33±3 year, BMI 22.7±0.6kg m^-2; mean±SE) underwent a 4hour IVGTT (0.3g kg¹ glucose enriched with 3-6% D-[U-¹³C]glucose and 5-10% 3-0-methyl-D-glucose) preceded by a 2hour investigation under basal conditions (5mg kg^-1 of D-[U-¹³C]glucose and 8mg kg^-1 of 3-0-methyl-D-glucose). A new model described the kinetics of the two glucose tracers and native glucose using a two-compartment structure for glucose and a one-compartment structure for insulin effects. Insulin sensitivities of distribution/transport, disposal, and EGP were similar (11.5±3.8 vs 10.4±3.9 vs 11.1±2.7x10^-2mL kg^-1 min^-1 per mU L^-1; P = NS, ANOVA). When expressed in terms of ability to lower glucose concetration, stimulation of disposal and stimulation of distribution/transport accounted each independently for 25% and 30% of the overall effect. Suppression of EGP was more effective (P < 0.01, ANOVA) and accounted for 50% of the overall effect. EGP was suppressed by 70 (52-82)% (95% CI; relative to basal) within 60min of the IVGTT; glucose distribution/transport was least responsive to insulin and was maximally activated by 62 (34-96)% above basal at 80min compared to maximum 279 (116-565)% activation of glucose disposal at 20min. The deactivation of glucose distribution/transport was slower than that of glucose disposal and EGP (P < 0.02) with halftimes of 207 (84-510)min, 12 (7-22)min, and 29 (16-54)min, respectively. The minimal model insulin sensitivity S_I was tightly correlated with and linearly related to sensitivity of EGP (r = 0.96, P < 0.005) and correlated positively but nonsignificantly with distribution/transport sensitivity (r = 0.73, P = 0.10) and disposal sensitivity (r = 0.55, P = 0.26). We conclude that in healthy subjects during an IVGTT, the two peripheral insulin effects account jointly for approximately half of the overall insulin-stimulated glucose lowering, each effect contributing equally. Suppression of endogenous glucose production matches the effect in the periphery.