The effect of dehydration on rectal temperature, respiratory frequency, upper respiratory evaporation, cardiac output, and common carotid blood flow was studied in large mongrel dogs at rest at ambient temperatures between 25 and 45 degrees C. Measurements were made in animals hydrated ad libitum and when they had been dehydrated by removal of drinking water. In hydrated animals, mean body weight was 31.6 +/- 1.7 (SE) kg and plasma osmolality was 296 +/- 2 mosmol/kg H2O. Dehydration decreased body weight to 28.2 +/- 1.5 kg and increased osmolality to 328 +/- 5 mosmol/kg H2O. At all ambient temperatures, every dehydrated animal had a higher rectal temperature, lower respiratory frequency, lower upper respiratory evaporation, lower cardiac output, and lower common carotid blood flow. Rectal temperature, measured in seven animals, was constant as ambient temperature was raised from 25 to 45 degrees C in both the hydrated and dehydrated state, but was elevated by 0.72 degrees C in dehydrated animals. Hypothalamic temperature, measured in two animals, was elevated less than rectal temperature when they were dehydrated. Upper respiratory evaporation and respiratory frequency, measured in seven animals, increased with increasing ambient temperature in both the hydrated and the dehydrated dogs, but were lower at every ambient temperature in dehydrated animals. Cardiac output, measured in five animals, was lower in dehydrated animals at every ambient temperature.(ABSTRACT TRUNCATED AT 250 WORDS)