Spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SHRSP) were treated with a combination of a beta 1-blocker (metoprolol) and a calcium antagonist (felodipine) from 1 to 4 or from 4 to 6 months of age. The main trunk and peripheral branches of the mesenteric arterial tree were fixed by immersion and embedded in plastic. The total length of the internal elastic membrane and the media area were measured in cross-sectioned arteries. The ratio between media thickness (m) and luminal radius (ri) was then determined for a calculated, standardized condition, assuming a smooth and circular internal elastic membrane. The treatment caused a significant decrease in blood pressure and a reduction in m/ri ratios in the mesenteric arterial trunk as well as in mesenteric arterial branches when initiated in young as well as in adult SHR and SHRSP, i.e. the therapy efficiently prevents as well as reverses hypertensive arterial changes. The m/ri ratios in the superior mesenteric arterial trunks were significantly smaller in treated 4-month-old SHR and SHRSP than in WKY, although their blood pressures were not fully normalized. There was a marked increase in luminal radius in the young treated rats, possibly secondary to a therapy-induced increase in splanchnic blood flow.