Renal vascular resistance is increased in essential hypertension, as in genetic models of hypertension. Here we review the evidence that this is at least in part due to structural changes in the afferent arterioles. Rat studies show that the renal afferent arteriole is structurally narrowed in young and adult spontaneously hypertensive rats (SHR). Furthermore, in the second generation of crossbred SHRs/normotensive rats (SHR/WKY F(2)-hybrids), a narrowed afferent arteriole lumen diameter at 7 weeks is a predictor of later development of high blood pressure. The reduced lumen diameter of resistance vessels is accompanied by a decrease in media cross-sectional area in SHR and could therefore be due to inhibited growth. Evidence from a primate model of hypertension has shown a negative correlation between left ventricular hypertrophy and afferent arteriole diameter, but apparently no relation to blood pressure. In SHR, the antihypertensive effect of angiotensin converting enzyme (ACE) inhibitors is mediated through renal vascular mechanisms, while ACE inhibitors (like AT(1) antagonists) have a more persistent effect on blood pressure after treatment withdrawal compared with other antihypertensive drugs. Taken together, the evidence suggests that structural narrowing of the renal afferent arteriole could be an important link in the pathogenesis of primary hypertension, at least in the SHR.