The sodium retaining effect of insulin was studied in ten Type 2 (non-insulin-dependent) diabetic patients (mean age 56 (43-73)years, mean body mass index 29.5 (24.2-33.7) kg/m o and eight age-matched control subjects (mean age 57 (43~58)years, mean body mass index 23.4 (20.8-26.6) kg/m2). The renal clearances of 99mTc-DTPA, lithium, sodium and potassium were measured over a basal period of 90 rain. Then insulin was infused at a rate of 40 mU-min-i, m-2. After an equilibration period of 90 min, the clearance measurements were repeated during a new 90 rain period. Blood glucose was clamped at the basal level (diabetic patients: 9.9 + 3.5, control subjects: 5.3 + 0.5 mmol/1) by a variable glucose infusion. Basal plasma insulin concentration was elevated in the diabetic patients (0.12 • 0.05 vs 0.05 • 0.02 pmol/ml,p < 0.01). Insulin infusion resulted in comparable absolute increments in plasma insulin concentrations in the diabetic group and in the control group (0.44 • 0.13 vs 0.36 + 0.07 pmol/ml, NS). The metabolic clearance rate of glucose during the last 30 min of insulin infusion was lower in the diabetic patients (155• vs 320 • 69 ml-min-~, m-2, p < 0.01), reflecting peripheral in-