TOTAL body-water in the intact animal is usually measured by dilution methods. In these, determinations are made of the volume of fluid in which a known amount of some freely-diffusible substance distributes itself in the body. Urea which, with the exception of some of the fluids of the kidney and the renal veins and the possible exception of the cerebrospinal fluid1 and blood cells2, is uniformly distributed in the body fluids, has recently been used for body-water measurements in dogs3, humans4,6, rats3and cats3.