Alterations in cerebral metabolic activity were measured after different doses of phenobarbital. Local cerebral glucose utilization was determined in 58 brain regions with the use of the [14C]deoxyglucose technique in 3-month-old Fischer 344 rats, at 1 h after the ip administration of saline or of phenobarbital. Whole brain glucose utilization declined in a dose-related manner by 4%, 13%, 33%, 35%, and 56% after phenobarbital 18, 60, 180, 300, and 600 mg/kg, respectively. The number of regions significantly affected (P< 0.05) increased from 7 to 95% of the regions examined between doses of 18 to 600 mg/kg. Metabolism decreased in all significantly affected regions except the interpeduncular nucleus, where it was increased. In a separate group of rats, the number of falls per 5 min from a constantly rotating cylinder was measured at subanesthetic doses of phenobarbital. Doses of drug that affected performance on the rotating cylinder (18 and 60 mg/kg) reduced glucose utilization in brain regions involved with motor performance, including the red nucleus, vestibular nucleus, substantia nigra, and deep layers of the superior colliculus, whereas cerebral cortical regions were not altered significantly. The results demonstrate that phenobarbital reduces cerebral glucose utilization, in a dose-dependent manner, in most brain regions and affects subcortical regions of the motor system significantly before reducing metabolism in the cerebral cortex.