Kidney composition and renal concentration ability in young rabbits.

1. The water, sodium, urea, potassium and ammonia concentrations of the kidney cortex and inner medulla of adult rabbits, and of rabbits 1, 4, 14 and 21 days old, were measured after mild dehydration. The urine osmolarity and serum and urine urea concentrations were also measured.2. Potassium and ammonia showed no consistent variation in the different parts of the kidney or at the different ages. Sodium concentrations expressed as m-equiv/100 g fresh weight were about twice as high in the medulla as in the cortex at all ages.3. The medulla/cortex ratio of urea concentration increased markedly between the fourteenth and twenty-first day after birth. So also did the urine osmolarity of the mildly dehydrated animals.4. The rise in the medulla/cortex urea ratio between birth and 21 days, and the even higher ratio found in adult animals, was due to a decrease in the concentration of urea in the cortex rather than to an increase in its concentration in the medulla.