Morphological changes were studied in 3 different strains of rats genetically hypertensive (LH) normotensive (LN), or with low blood pressure (LL). Renal lesions were evaluated at 3, 5, 9, and 21 weeks of age, when blood pressure was significantly different between the 3 groups and the difference increased up to 21 weeks of age. Small histological changes could be detected in the LH groups as early as the fifth week (2 cases out of 8). However the differences between hypertensive rats and normo or hypotensive rats were obvious only at 9 weeks progressively increasing up to the 21th week. The juxta glomerular apparatus was mainly affected with hypertrophy and hyalinisation and hyaline deposits within the wall of the afferent arteriole and lacis cells. The lesions were constant although focal and of moderate intensity. They were frequently associated with segmental hyalinisation of the glomerulus and slight tubular and interstitial changes. The comparative study between three strains of rats shows that the renal vascular lesions appear early but after the onset of hypertension.