The ratio between media thickenss and lumen radius was determined in cerebral arterial vessels of 15- and 200-day-old spontaneously hypertensive (SHR) and normotensive rats (Kyoto Wistar and local Wistar rats). A modification of Furuyama's morphometrical method was used. There was a statistically significant increase of media/radius ratio among medium size and large vessels in 15-day-old SHR. Furthermore, the media cross section area and lumen radius was increased in the internal carotid arteries of young SHR. These early vascular aberrations could be caused by the slight increase of blood pressure at this age or be due to other genetically determined mechanisms in SHR. In 200-day-old SHR a significantly increased media/radius ratio was seen in arterial vessels with a radius less than 80 microns when compared to local Wistar rats but only in the smaller arterioles (r less than 20 microns) when compared to Kyoto Wistar rats. The present results offer a likely explantation for the increased cerebrovascular resistance during maximum vasodilatation in SHR.