The distribution volume of [51Cr]EDTA, as an estimate of the extracellular fluid volume (ECV), glomerular filtration rate (GFR) and urinary excretion rate of endogenous creatinine (uc), as an index of muscle mass, were determined in obese patients before and after intestinal bypass operations. The results were compared to those in non-obese controls with the same age and height. GFR, ECV and uc were all significantly increased to the same extent (about 40%) in thirteen patients examined before operation (overweight 86-159%). Means of the ratio GFR/ECV and standard GFR (i.e. GFR corrected to a body surface area of 1.73 m2) did not differ from those in the controls. In eight patients examined before and 1 year after operation (body weight reduction 23-79 kg), GFR were unchanged and remained normal. ECV was significantly increased by 20% in nineteen patients investigated 1-7 years after operation (mean overweight 42%) whereas the mean of uc did not differ from that in the controls. Using the ratio GFR/ECV as reference for the function of the kidneys, the present study shows that the renal function in otherwise healthy obese subjects is normal throughout the whole range of overweight, and that standard GFR is a reliable parameter to assess the renal function even in patients with extreme obesity. The body weight reduction following intestinal bypass operation is in part due to fall in muscle mass, but the results suggest that a normal relation between body cell mass and body water is not achieved.