Since elderly people are prone to develop both hypo- and hyper-natraemia, we have investigated the biochemical and hormonal responses to overnight (9 h) abstinence from fluids and subsequent oral water load (20 ml/kg) in a group of healthy elderly (E) (mean age 68 years) and young (Y) (mean age 28 years) volunteers. The elderly subjects had significantly higher baseline plasma osmolality (E 293.5 +/- 0.5, Y 290.5 +/- 0.8 mOsm/kg, p < 0.05) but lower urinary osmolality (E 508 +/- 47, Y 842 +/- 52 mOsm/kg, p < 0.001) and lower plasma vasopressin (E 0.5 +/- 0.1, Y 2.3 +/- 0.6 pmol/l, p < 0.001) than the young. There was a significant difference in the mode of excretion, particularly maximum free water clearance (E 6.0 +/- 0.6, Y 10.1 +/- 0.8 ml/min) but no difference in the overall ability to excrete the water load (at 4 h E 93 +/- 8%, Y 92 +/- 5%, p > 0.05). The biochemical and hormonal results suggest that the elderly subjects were in a state similar to partial cranial diabetes insipidus which may predispose them to dehydration and hypernatraemia. The reduction in maximum free water clearance may predispose them to hyponatraemia if excess fluid is administered.