Methionine is a sulfur-containing amino acid which is reversibly converted into homocysteine. Homocysteine is an independent cardiovascular risk factor frequently associated with the insulinresistance syndrome. The effects of insulin on methionine and homocysteine kinetics in vivo are not known. Six middle-age male volunteers were infused with L-[methyl-²H₃, 1-¹³C]-methionine before (for 3 hours) and following (for 3 additional hours) an euglycemic, hyperinsulinemic (150 mU/L) clamp. Steady state methionine and homocysteine kinetics were determined using either plasma (i.e. those of methionine) or intracellular (i.e. those of plasma homocysteine) enrichments. Using plasma enrichments, insulin decreased methionine Ra (both methyl- and carbon Ra) by 25% (p<0.003 vs. basal), and methionine disposal into proteins by 50% (p<0.0005), whereas it increased homocysteine clearance by ~70% (p<0.025). Using intracellular enrichments, insulin increased all kinetic rates, mainly because homocysteine enrichment decreased by ~40% (p<0.001). In particular, transmethylation increased by 6-fold (p<0.02), transsulfuration by 4-fold (p=0.01), remethylation by 8-fold (p<0.025) and clearance by 8-fold (p<0.004). In summary: 1) Physiologic hyperinsulinemia stimulated homocysteine metabolic clearance irrespective of the model used; and, 2) Divergent changes in plasma methionine and homocysteine enrichments were observed following hyperinsulinemia, resulting in different changes on methionine and homocysteine kinetics. In conclusion, insulin increases homocysteine clearance in vivo and it may thus prevent homocysteine accumulation in body fluids. The use of plasma homocysteine as a surrogate of intracellular methionine enrichment, following acute perturbations such as insulin infusion, needs to be critically reassessed.