The effect of an eight-week treatment with the Ca2+ channel blocker nicardipine on different-sized pial arteries and intracerebral arteries was assessed in spontaneously hypertensive rats (SHR) by microanatomical techniques. Normotensive Wistar-Kyoto (WKY) rats were used as normotensive reference animals. In SHR a significant increase of systolic blood pressure (SBP) in comparison with WKY rats was noticeable. An increased thickness of tunica media and luminal narrowing were also seen in medium- and small-sized pial arteries, and in intracerebral arteries of SHR in comparison with WKY rats. The media-to-lumen ratio was also increased in medium (diameter between 150 and 50 microm) and small-sized (diameter < than 50 microm) pial and intracerebral arteries. Treatment with nicardipine significantly reduced SBP, the thickness of tunica media, media-to-lumen ratio and increased luminal area of medium- and small-sized pial arteries and of intracerebral arteries. These findings demonstrate that treatment of SHR with nicardipine induces a moderate vasodilatation of both pial and intracerebral arteries regulating cerebrovascular resistance. This property may be useful in avoiding generalized or exaggerated cerebrovascular dilatation in hypertension which could be accompanied by impaired brain perfusion.