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. Basal cerebral arteries were fixed by immersion and embedded in plastic. The ratio between media thickness and luminal radius was determined in cross-sectioned arteries for a standardized condition, assuming a smooth and circular internal elastic membrane. The treatment caused a significant decrease in blood pressure and a normalization of m/r ratios in basal cerebral arteries 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. Even though 4-month-old treated rats were still significantly hypertensive, their m/r ratios did not differ from those of normotensive controls. The latter results may indicate that the treatment affects the vessel walls also through other mechanisms than by decreasing the pressure load.