Halfway through the study, a section of the AOV, just caudal to its junction with the dorsal nasal vein, was extirpated on both sides. Before and after AOV surgery, the sheep were housed outdoors at 21-22°C and were exposed in a climatic chamber to daytime heat (40°C) and water deprivation for 5 days. In sheep outdoors, SBC was significantly lower after the AOV had been cut, with its 24-h mean reduced from 0.25 to 0.01°C (t 5 ϭ 3.06, P ϭ 0.03). Carotid blood temperature also was lower (by 0.28°C) at all times of day (t 5 ϭ 3.68, P ϭ 0.01), but the pattern of brain temperature was unchanged. The mean threshold temperature for SBC was not different before (38.85 Ϯ 0.28°C) and after (38.85 Ϯ 0.39°C) AOV surgery (t 5 ϭ0.00, P ϭ 1.00), but above the threshold, SBC magnitude was about twofold less after surgery. SBC after AOV surgery also was less during heat exposure and water deprivation. However, SBC increased progressively by the same magnitude (0.4°C) over the period of water deprivation, and return of drinking water led to rapid cessation of SBC in sheep before and after AOV surgery. We conclude that the AOV is not the only conduit for venous drainage contributing to SBC in sheep and that, contrary to widely held opinion, control of SBC does not involve changes in the vasomotor state of the AOV. brain temperature; evaporative heat loss; thermoregulation ARTIODACTYLS AND FELIDS HAVE the capacity to lower brain temperature below arterial blood temperature through a process termed selective brain cooling (SBC) (for reviews see Refs. 11 and 26). In artiodactyls, blood destined for the brain enters the carotid rete, a bilateral network of small arteries at the base of the brain embedded in the cavernous sinus, a venous lake containing relatively cool blood from the nasal mucosa and other areas of the head. Heat exchange between the warm arterial blood in the rete and the cool venous blood in the sinus cools cerebral arterial blood, so that brain temperature (measured near the hypothalamus) is lower than carotid arterial blood temperature (23). Historically, SBC has been investigated mainly in the laboratory, where it is conspicuous in hyperthermic animals (25). However, recent studies, particularly in free-living animals, have revealed that SBC is not exclusively under thermal control. Nonthermal inputs, particularly sympathetic nervous system activity, also influence its magnitude. While a resting, moderately hyperthermic antelope may exhibit SBC, intense exercise, for example, to escape predation, rapidly leads to a cessation of SBC and an increase in brain temperature (11,26).Largely as a result of an elegant series of experiments in reindeer (14 -16), it is widely held that the mechanism that allows SBC to be facilitated or inhibited involves relative blood flow in two routes of venous return from the nose (11,26). Cool blood drains from the nasal mucosa on each side of the head into the dorsal nasal vein, which in turn bifurcates into the angularis oculi vein (AOV) and facial vein. Blood that passes into the AOV...