A system for positron emission tomography study of conscious monkeys was newly developed. By use of this system in combination with a microdialysis technique, the effect of ketamine on the binding and release of dopamine was investigated. The administration of ketamine (5 mg/kg) caused sedation accompanied by psychotic symptoms such as nystagmus and stereotyped movements of extremities. During this psychotomimetic period produced by ketamine, a significant increase in the accumulation of the dopamine D2 receptor ligand N-[11C]methylspiperone was observed in the striatum compared with the level in the conscious state, while no significant change was observed in the frontal cortex and cerebellum. In contrast to the use of ketamine as the anesthetic, pentobarbital (25 mg/kg), which produced deeper anesthesia but no psychotic symptoms, caused a decrease in the accumulation of N-[11C]methylspiperone in the striatum. Kinetic analysis, conducted by a graphical method, revealed that the value of the association constant (K3) for N-[11C]methylspiperone binding in the striatum was increased to approximately 130% by ketamine and decreased to approximately 70% by pentobarbital compared with the control values. Furthermore, the release of dopamine from the striatum measured by microdialysis was not affected by ketamine anesthesia. These results indicate that ketamine facilitates striatal dopaminergic neurotransmission through increasing the binding activity of dopamine D2 receptors in the striatum, and suggest that these changes may be related to the psychotomimetic behavioral symptoms of this drug.