In vivo binding of 3-N-[11C]methylspiperone ([11C]NMSP) was saturable in the rat forebrain, but not in the cerebellum. Nonspecific binding was almost equivalent in all brain regions except for the white matter. [11C]NMSP binding was localized to receptor-rich fractions when low doses were administered (less than 20 nmol/kg body weight). The striatum-to-cerebellum ratio was a function of time after injection and administered dose. This ratio remained constant in low doses of under 30 nmol/kg. The radioactivity curve of the cerebellum in a control positron-emission tomographic study almost equaled that of the striatum in the dog pretreated with spiperone (2 mg). This indicates that the amount of binding in the cerebellum might be considered a nonspecific binding and unbound pool. The data obtained by the pretreatment study was different from that of displacement, which suggested that displaceable [11C]NMSP in the specific binding sites of the striatum was not completely cleared from the brain tissue by a large amount of unlabeled spiperone.