This study describes the development of the capacity to concentrate urine in 10- to 40-day-old rats that were normally weaned between 16 and 18 days, breast-fed until day 20, or abruptly weaned on day 16. Urine osmolarity after 24 h dehydration was significantly lower in 10- than in 20- and in 20- than in 40-day-old rats. The most pronounced increase occurred between 16 and 20 days in the three groups. The increase in urine osmolality was paralleled by an increase of papillary Na and urea concentrations. The length of the papilla increased linearly from day 10 to 40. Serum arginine vasopressin levels were not significantly different in dehydrated 10- and 20-day-old rats. Serum corticosterone increased significantly between 10 and 20 days of age in both normally weaned and breast-fed rats. The rise was accelerated between days 16 and 18 and paralleled the rise in urinary concentrating capacity. Adrenalectomy on day 16 abolished the increases in urine osmolality and papillary Na and urea concentrations, but not the growth of the papilla between days 16 and 20. The urinary concentrating capacity could be precociously induced by treatment with betamethasone from day 10 to 17 but not from day 17 to 20. Our results indicate that the accelerated development of urine concentrating capacity at the time of weaning is independent of dietary composition and most likely modulated by glucocorticoid hormones.