The present study was designed to assess the effect of long-term isradipine treatment on the morphology of different-sized coronary artery branches in spontaneously hypertensive rats (SHR). Male SHR (12-week-old) received 1 mg/kg/day isradipine or vehicle (control group) orally for 12 weeks. Age-matched normotensive Wistar-Kyoto (WKY) rats were used as a reference group and did not receive any treatment. The animals were perfused with a fixative solution through the left ventricle and left ventricle blocks were embedded in resin. Sections including different-sized coronary artery branches were examined under a light microscope connected with an image analyser. The area occupied by the medial layer and the wall-to-lumen ratio were assessed in coronary artery branches of small (diameter less than 100 microns), medium (diameter 100-250 microns) and large (diameter greater than 250 microns) size. In control SHR, the blood pressure values and morphometric parameters examined significantly increased (p less than 0.001) in comparison with normotensive WKY rats. Isradipine treatment normalized blood pressure values in SHR and significantly reduced the area occupied by the medial layer and the wall-to-lumen ratio in small and medium-sized, but not in large-sized, coronary artery branches. These results indicate that isradipine treatment is able not only to reduce blood pressure elevation in SHR, but also to counteract the hypertension-dependent changes in the morphology of arterial branches controlling coronary resistances.