To assess the mechanisms of insulin resistance following injury, we examined the relationship between insulin levels and glucose disposal in nine nonseptic, multiple trauma patients (average age 32 years, Injury Severity Score 22) five to 13 days postinjury. Fourteen age-matched normals served as controls. Using a modification of the euglycemic insulin clamp technique, insulin was infused in 35 two-hour studies using at least one of four infusions rates (0.5, 1.0, 2.0 or 5.0 mU/kg min). Basal glucose levels were maintained by a variable infusion of 20% dextrose using bedside glucose monitoring and a servo-control algorithm. The amount of glucose infused reflected glucose disposal (M, mg/kg.min). Tracer doses of (6,6,2D2) glucose were administered in selected subjects to determine endogenous glucose production. At plasma insulin concentrations less than 100 microU/ml, responses in both groups were similar. However, maximal glucose disposal rates were significantly less in the patients than in the controls (9.17 +/- 0.87 mg/kg . min vs. 14.3 +/- 0.78, mean +/- SEM, p less than 0.01). Insulin clearance rates in the patients were almost twice that seen in controls. To further characterize this decrease in insulin responsiveness, we studied six additional patients and 12 controls following the acute elevation of glucose 125 mg/dl above basal (hyperglycemic glucose clamp). In spite of exaggerated endogenous insulin production in the patients (80-200 microU/ml vs. 30--70 in controls), M was significantly lower (6.23 +/- 0.59 vs. 9.46 +/- 0.79, p less than 0.02). In conclusion, this study demonstrated that (1) the maximal rate of glucose disposal is reduced in trauma patients; (2) the metabolic clearance rate of insulin in the injured patients is almost twice normal and; (3) insulin resistance following injury appears to occur in peripheral tissues, probably skeletal muscle, and is consistent with a postreceptor defect.