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Division of Biomedical Sciences, University of California, Riverside 92521.

1. Measurement of rectal temperature (Tr), sweat rate, respiratory frequency (f) and respiratory evaporation (Eresp) were made in one Nubian and four Alpine-Toggenberg goats while they stood for 90 min in a climate chamber at 40 degrees C ambient temperature (Ta). The animals were studied when they were hydrated, when they had been dehydrated by 48 h water deprivation, and when they were rehydrated by voluntary drinking of water or saline or by intraruminal water administration. Plasma osmolality (Posm), plasma protein concentration (PP) and haematocrit (Hct) were measured before every experiment and before and after voluntary drinking. 2. Hydrated animals increased evaporation by panting and sweating during heat exposure and Tr rose about 1 degree C. The rate of sweating was as high or higher than Eresp. Dehydrated animals had lower sweat rates and higher Tr than hydrated animals, but f and Eresp were the same in hydrated and dehydrated animals. 3. When dehydrated goats were allowed to drink after 60 min of heat exposure, sweating began abruptly within 3 min of the start of drinking in every animal whether water or saline was drunk. Sweat rate returned to hydrated levels or higher before any change occurred in Posm, PP or Hct. Respiratory frequency was higher after drinking than in dehydrated animals which were not allowed to drink. 4. When water was administered by rumen tube after 60 min of heat exposure, sweating in the Nubian occurred with a short latency, similar to the onset after drinking. In the other four animals, sweating onset occurred on average at 13 min 42 s after intraruminal water administration. 5. It is concluded that sweating is a significant avenue of evaporative heat loss in these goats when they are hydrated and exposed to high Ta. Sweat rate is markedly reduced after water deprivation but returns to hydrated levels within 3 min after the start of drinking. The rapid recovery of sweating after voluntary drinking is not initiated by changes in Posm or in blood volume and does not appear to depend upon osmoreceptors in the mouth or gastrointestinal tract since it occurs after drinking either water or saline. The arrival of water in the rumen may be sufficient to initiate immediate sweating in some goats, but the act of drinking is necessary in others.

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