The purpose of this study was to examine effects of hypertension on mechanics of cerebral arterioles in nongenetic and genetic models of chronic hypertension. Pressure (servo null) and diameter were measured in pial arterioles of anesthetized renal hypertensive rats (one-kidney, one clip), uninephrectomized nonnotensive rats, spontaneously hypertensive rats, and nonnotensive Wistar-Kyoto rats. During maximal dilatation with EDTA, external diameter of pial arterioles at 70 mm Hg pial arteriolar pressure was not significantly different in renal hypertensive and nonnotensive rats (86±5 [mean±SEM] versus 84±4 μm) but was less in spontaneously hypertensive rats than in Wistar-Kyoto rats (81±3 versus 92± μm;p<;0.05). Cross-sectional area of the arteriolar wall (hlstological) was greater in renal hypertensive than in nonnotensive rats (1.360±131 versus 952±89 μm2;p<0.05) and in spontaneously hypertensive rats than in Wistar-Kyoto rats (1,294±97 versus 817±86μm2;p<0.05). The stress-strain relation obtained from pressure-diameter data during maximal dilatation with EDTA indicated that distensibility of pial arterioles, when fully relaxed, was greater in renal hypertensive and spontaneously hypertensive rats than in nonnotensive and Wistar-Kyoto rats. We used point-counting stereology to quantitate composition of pial arterioles in renal hypertensive rats. Cross-sectional area of smooth muscle and elastin was significantly greater in renal hypertensive than in nonnotensive rats (smooth muscle, 947±108 versus 620±62μm2; elastin, 101±11 versus 55±6 μm2;p<0.05), whereas cross-sectional area of collagen and basement membrane was not significantly different in the two groups (collagen, 6±1 versus 5±1 μm2; basement membrane, 120±12 versus 104±8 μm2). Thus, we conclude that 1) cerebral arterioles undergo hypertrophy in both renal hypertensive and spontaneously hypertensive rats; 2) cerebral arterioles in renal hypertensive rats do not undergo “remodeling” with a reduction in external diameter, whereas external diameter is smaller in spontaneously hypertensive than in Wistar-Kyoto rats; 3) distensibility of cerebral arterioles, when fully relaxed, is increased in renal hypertensive rats and is greater in spontaneously hypertensive than in Wistar-Kyoto rats; and 4) the distensible components of the arteriolar wall are increased disproportionately in cerebral arterioles of renal hypertensive rats, which may contribute to increases in arteriolar distensibility.