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Arteries from spontaneously hypertensive rats (SHR), stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar-Kyoto controls (WKY) were fixed by immersion and plastic embedded. The media cross-sectional area and the length of the internal elastic membrane were measured. The ratio between media thickness and internal radius was then calculated for a standardized condition, implying a perfectly smooth and circular internal elastic membrane. An increase of the media to radius ratio was already demonstrable in visceral arterial branches of SHR and SHRSP by 15 days of age, indicating that changes of arterial structure could be of pathogenetic importance in early hypertension. At seven months of age the ratio was significantly increased in mesenteric arterial branches of SHR and SHRSP and in renal arterial branches of SHRSP. The media to radius ratio was markedly increased in the renal and superior mesenteric arterial trunks of adult SHR (P<0.001 in both) and SHRSP (P<001 and P<0.01). This was due to a greater media cross-sectional area in the former vessels and to a combination of greater media and reduced radius in the latter.