Media/lumen area ratios and density of arteries and arterioles were obtained from tail sections of newborn and 2-week-old spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY) on prenatal/postnatal high salt diets or standard salt diets. Fixation was by a solution which caused minimal contraction and no differences in contractile response between SHR and WKY. Standard salt SHR of both ages demonstrated significantly greater media/lumen ratios in intermediate size arterioles but not in smaller or larger arterial categories. Prenatal high salt caused significant medial thickening in 49- to 58- micrometer arterioles of newborn SHR. Maintenance on high salt to 2 weeks caused significant medial thickening i SHR arterioles of 19-38 micrometer outside diameter. High salt significantly increased the density of terminal arterioles in 2-week SHR.