1. Measurements of brain and central blood temperature (T(br) and T(bl
)), metabolic rate (MR) and respiratory evaporative heat loss (REHL) w
ere made in trained goats walking on a treadmill at 4.8 km h-1 at trea
dmill inclines of 0, 5, 10, 15 and 20 % when they were fully hydrated
and at 0 % when they had been deprived of water for 72 h. 2. In hydrat
ed goats, exercise MR increased progressively with increasing treadmil
l incline. Both T(bl)), and T(br) rose during exercise, but T(bl) alwa
ys rose more than T(br) and selective brain cooling (SBC = T(bl)) - T(
br)) increased linearly with T(bl). Significant linear relationships w
ere also present between REHL and T(bl) and between SBC and REHL. Neit
her the slope of the regression relating SBC to T(bl) nor the threshol
d T(bl) for onset of SBC was affected by exercise intensity. Manual oc
clusion of the angularis oculi veins decreased SBC in a walking goat,
while occlusion of the facial veins increased SBC. 3. Dehydrated goats
had higher levels of T(bl), T(br) and SBC during exercise, but the re
lationship between SBC and T(bl) was the same in hydrated and dehydrat
ed animals. In dehydrated animals, REHL at a given T(bl) was lower and
SBC was thus maintained at reduced rates of REHL. 4. It is concluded
that SBC is a linear function of body core temperature in exercising g
oats and REHL appears to be a major factor underlying SBC in exercise.
The maintenance of SBC in spite of reduced REHL in dehydrated animals
could be a consequence of increased vascular resistance in the facial
vein and increased flow of cool nasal venous blood into the cranial c
avity.