The precision of a simplified single injection method for determination of the distribution volume of [51Cr] EDTA, as an estimate of the extracellular fluid volume (ECV), was determined in 49 patients (17 men and 32 women, aged 19-75 years) who had ECV determined twice with an interval of 1-5 days. The glomerular filtration rate (GFR) can be determined by the same method. The total day-to-day variation (coefficient of variation) of ECV was 11.4% and that of GFR 4.5%. ECV differed significantly between men and women so that ECV on average was 14% (P less than 0.001) and 7% (P less than 0.05) higher in men than in women for the same body weight and body surface area, respectively. Regression equations of ECV on body weight and body surface area in 44 normal men and in 40 normal women are presented.