Water intake and blood parameters of young (7-month) and old (23-month) male Brown Norway rats were assessed following a period of thermal dehydration. Rats of both ages were randomly assigned to one of three groups: (1) Unheated-blood sample, (2) Heated-blood sample, and (3) Heated-water intake. The colonic temperature of heated rats was raised at the rate of 0.05 degrees C/min for 1 h using an infrared heat lamp. Water intake was then measured over the following 2 h. The heating protocol resulted in a similar level of dehydration in both young and old rats; however, plasma osmolality and sodium concentration increased to a significant extent only in the young rats. Old rats drank significantly less water at all time points during the 2 h following the heat stress. While neither group replaced the water lost as a result of the thermal dehydration, the young rats did rehydrate to a greater extent. These results suggest that the diminished level of rehydration in aged rats, following a thermal dehydration, is due to an attenuated rise in plasma osmolality.