We studied insulin action at submaximal and maximal insulin levels in seven newly diagnosed type I (insulin-dependent) diabetic patients after 2 weeks ( t 1 2 ) and after 3 months (t3) of insulin treatment, and in seven control subjects. Insulin action was determined with a sequential euglycemic (5.0 mmol/L) glucose clamp technique using insulin infusion rates of 0.5, 1.0, 2.0, and 5.0 mU · kg −1 · min −1 in four periods of two hours each. The final 30 minutes of each infusion period (referred to as steady-state) were taken for the assessment of insulin action. Steady-state insulin levels were similar in the diabetic patients at t 1 2 and t3, and in control subjects. During the first and second infusion periods, steady-state glucose infusion rates (SSGIR) were lower at t 1 2 than at t3 (12.2 ± 1.7 v 18.8 ± 2.4, P < .05, and 34.3 ± 3.8 v 47.6 ± 2.5 μmol · kg −1 · min −1, P < .02, respectively), and were lower at t 1 2 compared to controls (12.2 ± 1.7 v 22.4 ± 2.3, P < .01, and 34.3 ± 3.8 v 47.3 ± 2.8 μmol · kg −1 · min −1, P < .02). No differences were found during the third and fourth infusion periods between t 1 2 and t3, or t 1 2 and controls. When these data were used to construct dose-response curves, insulin action was decreased in the diabetic patients at t 1 2 at submaximal insulin levels (shift to the right), while insulin responsiveness was unchanged. This finding may be regarded as a still-present manifestation of the metabolic derangement at the onset of the disease. Three months after initiating insulin treatment, insulin action is characterized by both normal sensitivity and responsiveness, although metabolic control was not normalized (HbA 1 9.3 ± 0.9% v 6.8 ± 0.3% in control subjects, P < .02). No significant correlation was found between metabolic control (glycosylated hemoglobin, HbA 1) and insulin action as measured by the SSGIR ( t 1 2 , −0.38 < r < 0.05; t3, −0.69 < r < −0.08 ), or the change in metabolic control v the change in insulin action ( t3 − t 1 2 , − .25 < r < .15 ). The normalization of insulin sensitivity coincided with a decrease in daily insulin requirement, whereas the β-cell reserve remained unchanged.