Six obese, non-insulin-dependent diabetic subjects were studied before and 3 mo after treatment with the sulfonylurea gliclazide, 40–80 mg b.i.d. Fasting plasma glucose fell significantly from 13.4 ±1.6 (SEM) to 8.6 ±1.2 mmol/L, accompanied by a significant reduction from 40.6 ± 3.7 to 29.8 ± 2.8 mM · h of the plasma glucose response to 75 g oral glucose. Fasting plasma insulin showed a nonsignificant increase from 24.8 ± 2.0 to 31.3 ± 2.3 mU/L. The percent specific binding of tracer 125I-insulin to erythrocytes and monocytes did not change significantly (from 9.8 ± 1.7 to 8.5 ± 0.7 for erythrocytes and 1.7 ± 0.3 to 1.6 ± 0.4 for monocytes). Glucose utilization was measured at three levels of insulin infusion (40, 100, and 300 mU/ kg/h) by the euglycemic clamp technique. Overall there was a significant (P < 0.05) increase in the disappearance rate (Rd) and metabolic clearance rate (MCRg) for glucose at the two higher insulin infusion rates (MCRg: 3.3 ± 0.7 to 5.1 ± 0.7 and 5.9 ± 0.9 to 7.9 ± 0.9 ml/kg/min), but not at the lowest infusion rate (MCRg: 3.6 ± 0.8 to 3.3 ± 0.6). Thus, the chronic hypoglycemic effect of gliclazide in obese diabetic subjects was associated with an improvement in insulin-mediated glucose utilization at high plasma insulin concentrations. This enhanced effect of insulin after gliclazide treatment was not accompanied by increased monocyte or erythrocyte insulin binding, which suggests that it was due to potentiation of postbinding insulin-sensitive pathways.