Recent experiments have shown that the kidney adapts to chronic variations in urine concentration. Glomerular filtration rate (GFR), kidney weight relative to body weight, thickness of inner stripe of the outer medulla, volume of epithelium in early thick ascending limb, and internephron heterogeneity are all decreased by chronic water diuresis and increased by chronic stimulation of urine concentration. It was further shown that the intrarenal pattern of hypertrophy observed after high protein (HP) intake, but not that observed after compensatory hypertrophy or normal growth with age, is exactly similar to that observed after chronic stimulation of urine concentration. Since solute-free water reabsorption (TcH2O) is markedly enhanced by HP diet, this suggests that the increases in GFR and renal mass observed after HP intake are, at least in part, an adaptive response of the kidney to increased urinary concentrating activity. The beneficial effects are induced by protein restriction in chronic renal failure (CRF) could thus be due, in part, to the reduction of this concentrating activity. This hypothesis was confirmed by an experiment performed in rats with experimental chronic renal failure (CRF) in which a chronic increase in water intake, reducing urine osmolality and TcH2O, without any change in food composition or consumption, reduced proteinuria, systemic hypertension, kidney hypertrophy, incidence of glomerulosclerosis, and mortality.