Ultrastructural changes in the arteriolar wall with caliber irregularity of spindle type, rosary type and diffuse dilatation type were studied in stroke-prone spontaneously hypertensive rats (SHRSP) aged 4, 6 and 10 months with systolic blood pressures of 210-240 mmHg by use of intravenously injected gelatin fluorescein preparation (IVGFP) and light and electron microscopy. Tridimensional graphic models were reconstructed from light micrographs of cross-sectioned serial preparations using a computerized micrograph-analyser. In the spindle type, I found markedly thickened media and smooth muscle cell necrosis at the narrowed portion of the arteriole central to the ampullar portion and a marked stretch and disappearance of the inner elastic lamina at the ampullar portion. These vascular changes of arteriolosclerosis were most advanced in the spindle type among three types of caliber irregularity. Tridimensional graphic models disclosed that there was no correlation between the wall thickening and luminal diameter of the arteriole. These results may explain the process of development of caliber irregularity in SHRSP.