The regulation of body water during hypotonic expansion (0.6% NaCl; 0.32 ml/kg/min i.v.) of the extracellular space was investigated in 16 male rabbits with different creatinine clearances due to variations in salt intake. In group A (n = 6) sodium intake was 43 mEq/kg/day, in group B (n = 10) 0.6 mEq/kg/day. After 2 weeks the exogenous creatinine clearance in group A averaged 3.85 ± SD 0.62 ml/min/kg, in group B 2.52 ± SD 0.58 ml/min/kg (p < 0.01). Comparable urine flows (group A: 0.40 ± SD 0.064 ml/min/kg, group B: 0.42 ± SD 0.094 ml/min/kg, p > 0.90) were not caused by differences in the free water clearance (group A: ± 0.13 ± SD 0.07 ml/min/kg, group B: ± 0.11 ± SD 0.05 ml/min/kg, p > 0.60), but by a different tubular reabsorption of sodium: group A: 95.1 ± SD 2.11%, group B: 89.4 ± SD 1.53% (p < 0.001). Our results suggest that a physiological mechanism controls the decreased tubular reabsorption of sodium in kidneys with reduced glomerular filtration rates during a rapid hypotonic expansion of the extracellular space.