Insulin resistance is associated with an increased risk of cardiovascular disease that is probably related to abnormalities of vascular wall function. The JCR:LA-cp rat is a unique animal model of human vascular disease that exhibits a profound insulin resistance, vasculopathy, and cardiovascular disease, and allows study of the relationships between insulin resistance and vascular function. Conductance and resistance arteries serve different functions, thus vascular disease may affect these types of artery differently. Concentration-response curves to norepinephrine, phenylephrine, and acetylcholine were studied in conductance vessels (aortic rings) and resistance vessels (mesenteric arteries) from 12-week-old male obese and lean JCR:LA-cp rats. The aortic rings and mesenteric arteries from obese rats showed increased maximal response to phenylephrine compared with those from lean rats, whereas only the mesenteric arteries from obese rats showed increased maximal response to norepinephrine. In aortic rings, relaxation to acetylcholine was similar for both genotypes, but the mesenteric arteries of obese rats showed impaired relaxation to acetylcholine. We conclude that the sensitivity to vasoconstriction is enhanced in aortic rings and mesenteric arteries of obese male JCR:LA-cp rats, but endothelial function is impaired only in the mesenteric resistance arteries of these animals. Hence functional aberrations in the obese, insulin-resistant state are more pronounced in mesenteric resistance arteries than in a major conductance artery.