We determined the structure of mesenteric small arteries after chronic elevation and chronic reduction of blood flow. In 6-wk-old rats, we ligated second-order side branches of every other first-order side branch of the superior mesenteric artery. This persistently reduced blood flow (-90%) in the vessels feeding into the ligated trees and elevated blood flow (+80%) in the nonligated mesenteric artery side branches. Four weeks after surgery, vessels that had been exposed to high blood flow (HF) or low blood flow (LF) and vessels from sham-operated rats (Sham) were isolated and mounted in a pressure myograph system. At an intraluminal pressure of 100 mmHg, the internal diameter at rest was larger in HF (533 +/- 23 microm) and smaller in LF (262 +/- 14 microm) than in Sham vessels (427 +/- 15 microm). Also, wall and media cross-sectional areas were larger in HF and smaller in LF than in Sham vessels (media: 22 +/- 1, 11 +/- 2, and 16 +/- 1 x 10(3) microm2, respectively), but circumferential wall stress did not differ among groups. DNA content was significantly increased in HF vessels (+100%) and was not modified in LF vessels. Maximal vasoconstrictions elicited by high potassium or norepinephrine were slightly increased in HF vessels but were reduced by 50% in LF vessels. Thus chronic changes in blood flow give rise to structural changes that normalize circumferential wall stress. Elevated blood flow resulted in outward hypertrophic remodeling involving hyperplasia. Reduced blood flow resulted in inward hypotrophic remodeling accompanied by hyporeactivity of the arterial smooth muscle.