In response to water deprivation or heat stress, healthy elderly men experience reduced thirst compared with a young control group. Because water deprivation and heat stress produce hypertonicity as well as reduced extracellular fluid volume, the aim of this study was to determine whether the elderly also experience reduced thirst in response to hypertonicity without extracellular fluid volume depletion. To achieve this, after an overnight fast, healthy community-dwelling nonsmoking old (65-78 yr, n = 7) and young (20-32 yr, n = 7) men received 120-min hypertonic (0.855 M) and isotonic (0.154 M) saline infusions (0.06 ml· min-1·kg-1) in double-blind randomized order at least 2 wk apart. Both groups had increased thirst (P < 0.001) and water intake (P < 0.05) after the hypertonic saline infusion compared with the isotonic saline infusion. However, despite similar plasma sodium increases and less plasma volume expansion during the hypertonic saline infusion compared with the young group, the old group was less thirsty (P < 0.01) and drank less room-temperature tap water (young 11.1 ± 1.2 ml/kg, old 3.9 ± 0.6 ml/kg, P < 0.001). This reduced thirst in the old group seemed to be mainly due to a lower thirst sensitivity (P < 0.05) to hypertonicity, although there was also a tendency toward an increased thirst threshold. Plasma arginine vasopressin increases were similar for the old and young groups with hypertonic saline, without significant changes during the isotonic saline infusion in either group.