The present study was designed to investigate whether the limited capacity for concentrating urine in neonatal rats is associated with an immature ability to regulate serum osmolality. During milk deprivation, the percent of reduction in body weight per 10 h (mean±SE) was 4.3±0.2, 3.7±0.1, 4.8±0.2, and 6.0±0.1% in 4-, 7-, 10-, and 14-d-old rats, respectively (n=23-24, each age). The osmolality of urine increased to 718±12 (4d), 741±28 (7d), 792±20 (10d), and 1, 203±41 mosmol/kg H₂O (14d). Free-water absorption (T^c_H2O) Promptly increased after deprivation of milk: It significantly increased from 2.3±0.3 (0-4h) to 3.4±0.1 (4-7h) (4d), from 3.1±0.3 to 4.1±0.3 (7d), from 3.6±0.4 to 5.2±0.3 (10d), and from 5.0±0.4 to 7.9±0.7 μl/min/100g (14d). The raised values were maintained at the later period of dehydration. Thus serum osmolality was unchanged throughout dehydration: 287±1.0 (7d) and 292±0.9 mosmol/kg H₂O (14d). On the other hand, the level of serum sodium concentration slightly but significantly increased (r=0.61) when the body weight reduction was higher than 5% of the control (14-d-old rats). These results indicate that neonatal rats of 4-14d control their serum osmolality by reabsorbing free water in the kidney during the 10 or 12h of milk deprivation.