In genetically hypertensive rats of the Milan hypertensive strain (MHS) and normotensive rats (NR) developed from the same Wistar stock colony, blood pressure (BP), plasma renin activity (PRA), sodium balance, and water balance were measured from the time of weaning to the seventh week postweaning in three separate but essentially identical experiments. In a fourth experiment, total and extracellular water, total sodium, and exchangeable sodium were measured in MHS and NR at 24 and 130 days of age. Although the time course of changes varied slightly between experiments, BP of both NR and MHS rose until the second and third weeks postweaning, at which time BP in MHS was 40 to 50 mm Hg higher than in NR. PRA in MHS was one-half that of NR at weaning. Increasing BP was accompanied by falling PRA in both, and PRA was not significantly different when stable, adult blood pressure was reached. Urinary volume in MHS was 50% to 100% greater (P less than 0.001) than in NR at weaning and for a few days after. Sodium was retained to a greater extent by MHS during the period when the blood pressure difference develops, from weaning to the fourth week postweaning. This sodium retention (MHS = 97.0 plus or minus 10.3, NR = 65.2 plus or minus 6.8 SE mu-Eq Na retained/g body weight gain; P less than 0.005) is the result of significantly lower urinary excretion of dietary sodium by MHS. A causative role for the kidney is suggested in the established of high blood pressure in MHS.