OBJECTIVE: Angiotensin-converting enzyme inhibitors reduce cardiovascular mortality and improve endothelial function in type 2 diabetic patients. We hypothesized that 2 months of quinapril treatment would improve insulin-stimulated endothelial function and glucose uptake in type 2 diabetic subjects and simultaneously increase the expression of genes that are pertinent for endothelial function and metabolism. METHODS: Twenty-four type 2 diabetic subjects were randomized to receive 2 months of quinapril 20 mg daily or no treatment in an open parallel study. Endothelium-dependent and -independent vasodilation was studied during serotonin or sodium nitroprusside infusion in the diabetic patients and in 15 healthy subjects. Endothelial function, insulin-stimulated endothelial function, and insulin-stimulated glucose uptake were measured before and after quinapril treatment. Blood flow was measured by venous occlusion plethysmography. Gene expression was measured by real-time PCR. RESULTS: Quinapril treatment increased insulin-stimulated endothelial function in the type 2 diabetic subjects (P = 0.005), whereas forearm glucose uptake was unchanged. Endothelial function was also increased by quinapril (P = 0.001). Systolic and diastolic blood pressures were reduced by quinapril (P < 0.001). Quinapril increased adiponectin gene expression in vascular tissue obtained from sc adipose biopsies. CONCLUSIONS: Quinapril treatment increases insulin-stimulated endothelial function in patients with type 2 diabetes. Increased vascular adiponectin gene expression may contribute to this beneficial effect.