We have previously shown that patients with hypercholesterolemia have a depressed endothelium response to acetylcholine, which is related to an impaired release of nitric oxide from the endothelium. To determine whether such abnormal response is due to a decreased availability of L- arginine, the substrate for nitric oxide production by the endothelium, we studied the response to acetylcholine before and after administration of L-arginine in 12 hypercholesterolemic patients (6 male; age 59+/-9 years), in 12 controls (7 males; age 49+/-7 years). Drugs were infused into the brachial artery and forearm blood flow was measured by a strain gauge plethysmography. We also studied the effect of L-arginine on the response to the endothelium-independent sodium nitroprusside and the effect of D-arginine, the isoform of arginine which is not a precursor for nitric oxide synthesis, on the response to acetylcholine. As shown previously, the vasodilation in response to acetylcholine was significantly reduced in hypercholesteremic patients compared to normal controls. L-arginine significantly potentiated the response to acetylcholine in normal controls. However, no difference in the vasodilator effect of acetylcholine was observed after administration of L-arginine in hypercholesterolemic patients. L-arginine did not modify the vascular responses to sodium nitroprusside in either group. D- arginine did not modify the response to acetylcholine in either group. Thus, these findings indicate that patients with hypercholesterolemia failed to show the normal augmentation in endothelium-dependent responses after administration of L-arginine, the natural substrate for endothelial synthesis of nitric oxide. These findings indicate that the impaired endothelium-dependent vascular responses of hypercholesterolemic patients is not related to a decreased availability of nitric oxide substrate.