Infection results in a state of insulin resistance, but the pathogenesis is poorly understood. Intravenous administration of bacterial lipopolysaccharide (LPS) has been used to mimic the febrile and systemic inflammatory responses to infection, but it is unclear whether LPS induces insulin resistance in man. To investigate the effects of LPS on insulin sensitivity and substrate utilization, we administered, in paired cross-over studies, either 20 U/kg Escherichia coli endotoxin or saline control to healthy volunteers (n = 6) 120 min after the start of a 10-h euglycemic hyperinsulinemic clamp (insulin infusion rate, 80 mU/m²·min). LPS induced a fever, tachycardia, and mild arterial hypotension. Glucose utilization increased abruptly 120 min after LPS administration (+64.1 ± 12.0%; P< 0.003), but then declined progressively, and insulin resistance was evident by 420 min (+1.9 ± 3.5%; P < 0.05). The reduction in glucose utilization, like that observed in sepsis, was related to impaired nonoxidative glucose disposal and not abnormal glucose oxidation. The cortisol and GH responses to LPS were of sufficient duration and magnitude to explain the insulin resistance. LPS administration results in metabolic responses very similar to those observed in sepsis and could provide a useful model for the study of insulin resistance in human critical illness.