From measurements in eighty-six adults it is shown how the distribution volume of [51Cr]EDTA, as an estimate of the extracellar fluid volume (ECV), can be determined accurately using a simplified single injection method. The method only needs drawing of a few blood samples 3-5 h after a quantitative i.v. injection of [51Cr]EDTA and measurement of the subject's body weight. The glomerular filtration rate (GFR) can be calculated from the same few data, and with it, the ratio ECV/GFR or the reverse ratio GFR/ECV. These easily determined ratios might be relevant measurements of the kidney's main function: regulation of composition and size of the extracellular fluids, since they indicate how often 'that which is to be regulated' (i.e. the extracellular fluids) comes into contact with 'the regulator' (i.e. the kidney).