OBJECTIVE Evaluation of the time required until a change in the basal insulin infusion rate with an insulin pump induces subsequent changes in the metabolic effect. RESEARCH DESIGN AND METHODS In this euglycemic glucose clamp study, 10 male subjects with type 1 diabetes received three different subcutaneous insulin infusion rates (0.5, 1.0, and 2.0 units/h; for 4 h each) of insulin lispro (IL) with insulin pumps. RESULTS An increase in insulinemia occurred within 15-30 min after changing the infusion rate. While the serum IL levels reached a steady state at the end of the infusion period, the glucose infusion rates did not always reach steady-state levels with the higher infusion rates. However, an increase in the glucose consumption occurred within 30-60 min after switching the infusion rate. CONCLUSIONS Several hours are required until a new steady state in the metabolic effect is achieved after a significant change in basal insulin infusion.