Panting is a mechanism that increases respiratory evaporative heat loss (REHL) under heat load. Because REHL uses body water, it is physiologically and ecologically relevant to know under what conditions free-ranging animals use panting. We investigated whether the cranial arterio-venous temperature difference could provide information about REHL. We exposed sheep to environments varying in ambient dry bulb temperatures (Env 1: ~15°C, Env 2: ~25°C, Env 3: ~40°C, Env 4: ~40°C + infrared radiation) and measured REHL simultaneously with carotid arterial (T (car)) and jugular venous (T (jug)) blood temperatures, as well as brain (T (brain)) and rectal (T (rec)) temperatures. REHL increased significantly with ambient temperature, from 18.4 ± 4.5 W at Env 1 to 79.5 ± 12.6 W at Env 4 (P < 10(-6)). While there was no effect of environment on T (car) (P = 0.7) or T (jug) (P = 0.09), the difference between them (T (a-v) = T (car) - T (jug)) increased from Env 1 to Env 2 (P = 0.04) and from Env 3 to Env 4 (P = 0.008). T (a-v) reached a maximum of 0.7 ± 0.2°C at Env 4 and was positively correlated with REHL across environments (r (2) = 0.78, F = 34.7, P < 10(-3)). Calculated cranial blood flow changed only from Env 2 to Env 3 (P = 0.002). The increase in REHL maintained homeothermy when dry heat loss decreased. While REHL could increase without generating an increase in T (a-v), any increase in T (a-v) was always associated with an increase in REHL. We conclude that the cranial T (a-v) provides useful information about REHL in panting animals.