The time courses of drug concentration in serum after i.v. drip infusion of 2 mg/kg of tobramycin and 25 mg/kg of cefazolin in children were analyzed by model-independent moment analysis. The volume of distribution at the steady state per body weight (Vdss/BW) of tobramycin was in the range of 212 to 335 ml/kg and that of cefazolin was 119 to 156 ml/kg. A plot of the differences of Vdss/BW obtained in the same child for tobramycin and cefazolin against the value of Vdss/BW of tobramycin gave a linear regression line (r = 0.971). The magnitude of Vdss (1) of tobramycin could be well interpreted as corresponding to the extracellular water volume. In the case of cefazolin, the extracellular water space accounts for about 60% of the total distribution volume. The remaining 40% of the total Vdss of cefazolin was considered to be accounted for by the disposing organs.