Because the rabbit kidney is being used as an experimental model with increasing frequency, this study was designed to measure the relationships between cell number, size, and contents in the developing rabbit. Kidney slice extracellular and intracellular fluid spaces, high in the fetus and neonate, declined as the rabbits matured, being paralleled by changes in body fluid spaces. Although the cellular contents of both sodium and potassium were increased in the young kidney, intracellular sodium concentration was slightly lower in the fetal (43.6 mEq/liter) and 2-wk kidneys (44.5 mEq/liter) than in the mature kidney (51.7 mEq/liter). Intracellular potassium concentrations were similar in all age groups (163 to 167 mEq/liter). Tissue protein content was similar during development. In contrast, DNA content and the number of nuclei in kidney tissue were high in the fetus (DNA, 59.9 mg/g solids; nuclei, 3.9 x 109/g solids), decreasing in postnatal life (DNA in adult, 18.2 mg/g solids; nuclei in adult, 1.0 x 109/g solids). In association with this, the diameter of proximal tubular cells increased with maturation. These data should be valuable to those interested in kidney development.