Body temperature, plasma responses, and subjective ratings of thirst and hotness were studied in 5 older men (OM, 61-67 yr) and 6 younger men (YM, 21-29 yr) during 180-min thermal dehydration and subsequent 60-min rehydration (45 degrees C, 25% relative humidity). Rectal temperature (Tre) increased more rapidly and to a greater magnitude in OM, while average total body sweat rates and chest sweat rates were not significantly different. During dehydration, both OM and YM lost similar body weight (1.52 +/- 0.11 vs. 1.55 +/- 0.22%, mean +/- SE). However, in OM, plasma volume (Pv) decreased (-11.3 +/- 2.8 vs. -4.9 +/- 2.9%), and plasma osmolality (Posm) increased to a greater extent (+5.0 +/- 0.32 vs. 1.1 +/- 0.24 mosmol/kg) compared with YM. On rehydration, OM and YM similarly replaced water debt (46.6 +/- 4.9 vs. 49.0 +/- 3.0%). Within 30 min of drinking, YM had restored Pv and Posm, whereas OM showed slower responses, restoring Posm after 60 min and Pv only after a subsequent 30 min at 25 degrees C. Despite a higher Tre and greater change in Pv and Posm, OM rated themselves less thirsty and not significantly hotter than YM. These findings suggest that aging results in decreased ability to maintain Tre during heat stress and that the mechanisms comprise a combination of alterations in body fluid distribution and perception.