The renal and vasopressin (AVP) response to a standard oral water load (20 ml/kg) was examined in a group of water-replete healthy elderly men (n=6). Two groups, respectively, of water-replete and water-deprived young healthy volunteers acted as controls. After 2h, the old group had excreted 41±2.4% (mean±SEM) of the water load compared to 100.7±8.8% in the water-replete young group and 70±3.8% in the water-deprived young group (P<0.01). Similarly, peak diuresis (7.01±0.48 ml/kg) and peak free-water clearance (5.7±0.48ml/min) as determined from hourly sampling in the old group were delayed and significantly less than both young groups (P<0.01) (peak diuresis, young water-replete, 10.86±0.56ml/kg, young water-deprived, 10.2±0.64 ml/kg, peak free-water clearance, young water-replete 8.4±0.72ml/min, young water-deprived 9.5±0.88ml/min). When these indices were adjusted for reduced creatinine clearance (Cer) in the elderly, there was no significant difference between the young and old groups. Plasma AVP decreased similarly in all three groups following ingestion of water but there was no significant difference in mean plasma AVP between the young and old subjects throughout the study period. We therefore conclude that ability to excrete excess water promptly is impaired in healthy elderly men. This defect is due, at least in part, to an age-related reduction in glomerular filtration rate.