Summary Temperature measurements were made simultaneously in the arterial blood and various brain structures in 5 chronically prepared rhesus monkeys at rest in a lighted, sound-attenuated, thermoregulated chamber. The level of alertness of the animals was also noted by observing changes in overt behavior together with EEG and EMG recordings. Spontaneous and induced shifts in the level of arousal were followed in seconds by warming or cooling of the arterial blood and seconds to minutes later by parallel shifts in the temperature of the majority of the brain sites. Arterial blood temperature was thus found to yield a sensitive index of the behavioral state of the monkey in relation to sleep and arousal. Changes in the mean temperature of the arterial blood going to the brain appeared primarily responsible for temperature fluctuations in the brain. A number of steadily maintained but differing thermal gradients were found to exist between regional brain sites and the arterial blood.