Five days fasting in 12 non-obese, healthy volunteers caused a reduced capacity to excrete a single water load and to form maximally hypotonic urine. The minimum value of osmolarity of urine after fasting was on average 113mOsm/l, while it was 63 mOsm/l under control conditions. This abnormality in the formation of hypotonic urine could not be explained as the consequence of reduced glomerular filtration or enhanced solute excretion. The reduced diluting capacity is associated with an inadequate reduction of tubular water reabsorption.