Previous investigations in normal humans and rats have shown an increase in insulin sensitivity and binding affinity of adipocytes isolated 1-3 h after glucose ingestion. To determine whether a rapid enhancement of the action of insulin follows glucose ingestion in vivo, the present studies have utilized 120-min 20 mU/m2 X min euglycemic insulin infusions before and after 7.5-, 15-, 25-, and 100-g oral glucose loads. Euglycemic insulin infusions after the carbohydrate challenge were begun after arterialized blood glucose and insulin values had returned to baseline. After 15- and 25-g oral glucose loads during the 20-120-min interval of insulin infusion, glucose infusion rates increased by 44 +/- 6% (P less than 0.0001) and 47 +/- 9% (P less than 0.0002), respectively. No significant differences in arterialized glucose or insulin values existed between basal and post-glucose insulin infusions. In addition, no significant differences in hepatic glucose production or counter-regulatory hormone levels were found between basal and post-glucose insulin infusions. Control infusion studies including subjects who ingested saline or mannitol failed to show an increase in insulin action. Studies were carried out to mimic the insulin curve seen after 15- and 25-g oral glucose loads. Euglycemic insulin infusions after these insulin simulation studies show a 34 +/- 7% enhancement compared to baseline euglycemic insulin infusions. These results demonstrate a rapid enhancement of insulin action after oral glucose challenge in normal humans. The insulin simulation studies suggest that insulin itself either directly or through release of another factor acts on muscle to increase insulin sensitivity. The increase in insulin action demonstrated in these investigations may represent an important regulatory mechanism to modulate tissue insulin sensitivity.