Blood and extracellular fluid volumes were measured in four adult patients with Bartter's syndrome and compared with those of 21 healthy control subjects. Extracellular fluid volumes were significantly lower than in the control group (-7%), whereas blood volumes were within the normal range. Consequently, the ratio of blood volume to interstitial fluid volume was significantly elevated (0.42 v normal 0.35). The results are consistent with the concept that a tubular reabsorption defect is present in Bartter's syndrome.