The present positron emission tomography study used 2-deoxy-2[18F]fluoro-D-glucose to examine age-related changes in local cerebral metabolic rates for glucose (LCMRglc) in sedated rhesus macaques and vervet monkeys. Nineteen vervet monkeys were scanned in a cross-sectional design, which consisted of three age groups (birth to 59 d, 60–179 d, 180 d to adult) that captured the developmental period of greatest synaptic density within the second group. Two rhesus and two vervet monkeys were also examined longitudinally. Subjects were sedated throughout the procedure with a combination of ketamine and midazolam. Longitudinal and cross-sectional analyses of 20 brain regions indicated lowest LCMRglc in rhesus and vervet monkeys <2 months of age (about 60% of adult levels). These metabolic rates more than doubled to approximately 155% of adult levels after the second postnatal month, coincident with transient synaptic overproduction and increased sociobehavioral activity. LCMRglc remained high until 6 months postnatally, coincident with continued dendritic growth and the emerging sociobehavioral independence of the young monkey. After 6 months, LCMRglc decreased gradually to adult levels. Thus, the period between 2 and 6 months is a time of peak metabolic activity, which coincides with increased histologic and behavioral activity in the developing monkey.