1. Larger arterial vessels of 15-day-old spontaneously hypertensive (SH) rats showed significantly greater media/internal radius ratio than those of Wistar—Kyoto rats in the brain, kidney and mesentery.
2. Since larger cerebral and renal arteries of adult SH rats did not differ from controls the above results indicate the presence of a non-pressure-dependent vascular abnormality in young SH rats.
3. There were marked differences between the vascular beds of adult SH rats in the increase of media/radius ratio. Thus the changes were evident in all sizes of mesenteric arteries but only in the smallest cerebral arteries.