An in situ model was developed for studying mechanical properties of mesenteric arteries in rats. A branch of the mesenteric artery was exposed and dissected in normotensive (WKY) and spontaneously hypertensive rats (SHR). A catheter was introduced into the larger branch of the mesenteric artery and connected to a pressure chamber. The artery was submitted to transmural pressures ranging from 0 to 200 mmHg per steps of 25 mmHg and observed using a microscope-video-camera system. The diameter-pressure relations were established under basal conditions, under contraction (phenylephrine 10(-6) M), and after abolition of the smooth muscle tone by potassium cyanide (KCN, 0.1 mg/mL). The arterial segment was then fixed (glutaraldehyde 2.5%), and the wall cross-sectional areas were measured in transverse sections. Compliances, distensibility, wall tensions, and wall stresses were calculated from diameter, pressure, and media thickness values under three conditions. Active tension and active stress were defined as differences in wall stresses and wall tensions calculated under passive and active conditions. Comparison of WKY and SHR when arteries were studied at the respective operating pressure indicates (1) thicker and stiffer mesenteric arteries in SHRs than in WKY rats, (2) similar wall stresses in mesenteric arteries from WKY and SHRs despite larger wall tensions in the hypertensive group, and (3) larger contractility to phenylephrine in SHRs than in WKY mesenteric arteries.