The aim of the study was to find a possible explanation for the development of hypertension in two genetically hypertensive rat strains. Isolated kidneys from spontaneously hypertensive rats (SHR) and Dahl salt-sensitive rats (Dahl-S) kept on a low-salt diet were perfused at normotensive pressure (110 mm Hg). In adult (12-week-old) rats, no significant change in either parameter of kidney function was found in the two strains. In contrast, substantial changes were observed in both strains at the age of 6 weeks. Glomerular filtration rate and filtration fraction were similarly reduced both in SHR and Dahl-S rats. In SHR, the amount of sodium excreted was reduced in parallel to the GFR decrease. Sodium excretion was lower in Dahl-S than in Dahl-R due to the former higher Na<sup>+</sup> tubular reabsorption. The same was true for water excretion and its tubular reabsorption in this strain. In conclusion, the decreased amount of sodium excreted found in young SHR and Dahl-S might contribute to the development of hypertension in these strains. However, the reasons for this reduction are different in these two strains. In SHR, it results from decreased GFR whereas, in Dahl-S it is mainly due to increased tubular reabsorption.