Fructose 2,6-bisphosphate (Fru-2,6-P2) levels and 6-phosphofructo-1-kinase and 6-phosphofructo-2-kinase activities have been studied in rat brain during development from embryonal to adult state. Fru-2,6-P2 increases slightly from day 16 of gestation, reaching a maximum 24 h after birth, remaining quite constant during postnatal development. In contrast with 6-phosphofructo-1-kinase, which increases progressively after the first week of age, 6-phosphofructo-2-kinase remains unaltered throughout the period studied. The role of Fru-2,6-P2 in controlling cerebral glycolysis is discussed.