Marsupials reportedly can implement selectivebrain cooling despite lacking a carotid rete. We measuredbrain (hypothalamic) and carotid arterial blood temperaturesevery 5 min for 5, 17, and 63 days in spring in threefree-living western grey kangaroos. Body temperature washighest during the night, and decreased rapidly early in themorning, reaching a nadir at 10:00. The highest body temperaturesrecorded occurred sporadically in the afternoon,presumably associated with exercise. Hypothalamic temperatureconsistently exceeded arterial blood temperature,by an average 0.3°C, except during these afternoon eventswhen hypothalamic temperature lagged behind, and wasoccasionally lower than, the simultaneous arterial bloodtemperature. The reversal in temperatures resulted from thethermal inertia of the brain; changes in the brain to arterialblood temperature diVerence were related to the rate ofchange of arterial blood temperature on both heating andcooling (P <0.001 for all three kangaroos). We concludethat these data are not evidence for active selective braincooling in kangaroos. The eVect of thermal inertia on braintemperature is larger than might be expected in the greykangaroo, a discrepancy that we speculate derives from theunique vascular anatomy of the marsupial brain.