We investigated the morphological (wall-lumen ratio) and contractile characteristics of distal mesenteric arteries from spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) controls at a distending pressure of 63% of the mean aortic pressure of each rat using a pressure arteriograph. The wall-lumen ratios obtained were compared with those obtained at a pressure of 100 mm Hg. Experiments were carried out at 5 and 20 weeks. Mean aortic pressure of SHR was significantly increased at 5 weeks compared with that of WKY and was further increased by 20 weeks. At 63% of mean aortic pressure, no difference in the wall-lumen ratio of the arteries was observed between strains at 5 weeks; at 20 weeks, the wall-lumen ratio of SHR arteries was significantly increased compared that in WKY arteries. The wall-lumen ratio of SHR vessels did not differ at 63% mean aortic pressure compared with 100 mm Hg at either 5 or 20 weeks, whereas this parameter was significantly reduced in WKY vessels at 100 mm Hg compared with 63% mean aortic pressure at 5 and 20 weeks. In the presence of spontaneous myogenic tone, there was a borderline reduction in the lumen diameter of SHR vessels compared with WKY vessels and with increasing norepinephrine concentrations at 5 weeks. At 20 weeks, lumen diameter between strains did not differ in the presence of myogenic tone nor with increasing norepinephrine concentrations. Similar results were obtained when vessels from both rat strains were pressurized to 80 mm Hg. Thus, the increased wall-lumen ratio in the distal mesenteric arteries from adult SHR compared with those from WKY is not associated with an increased contractility under isobaric conditions when studied at physiological distending pressure. (Hypertension. 1996;28:604-608.)