Human neuroimaging studies have demonstrated the presence of a "default system" in the brain, which shows a "default mode of brain activity," i.e., greater activity during the resting state than during an attention-demanding cognitive task. The default system mainly involves the medial prefrontal and medial parietal areas, including the anterior and posterior cingulate cortex. It has been proposed that this default activity is concerned with internal thought processes. Recently, it has been indicated that chimpanzees show high metabolic levels in these medial brain areas during rest. Correlated low-frequency spontaneous activity as measured by functional magnetic resonance imaging was observed between the medial parietal and medial prefrontal areas in the anesthetized monkey. However, there have been few attempts to demonstrate a default system that shows task-induced deactivation in nonhuman primates. We conducted a positron emission tomography study with [(15)O]H(2)O to demonstrate a default mode of brain activity in the awake monkey sitting on a primate chair. Macaque monkeys showed higher level of regional blood flow in these medial brain areas as well as lateral and orbital prefrontal areas during rest compared with that under a working memory task, suggesting the existence of internal thought processes in the monkey. However, during rest in the monkey, the highest level of blood flow relative to that in other brain regions was observed not in the default system but in the dorsal striatum, suggesting that regions with the highest cerebral blood flow during rest may differ depending on the resting condition and/or species.