Seven-day-old rats (15 g body weight) were injected subcutaneously with iodo-[14C] antipyrine (5 µCi). After a variable period, each pup was decapitated and arterial blood collected for scintillation counting. Brains were immediately removed and either prepared for isotopic counting or frozen for autoradiography. The brain:blood partition coefficient was determined [0.944 ± 0.006 ml/g (mean ± SEM)]. Both cerebral blood flow (CBF) and regional CBF (RCBF) were calculated according to a formula derived from the Fick equation. CBF equaled 66 ± 4 ml/100 g/min (mean ± SEM), a value midway between reported 1-day-old rat CBF and adult rat CBF. Autoradiographs were of sufficient quality to permit microdensitimetric readings of a minimum of 11 structures. RCBF ranged from 20 ml/100 g/min in subcortical white matter to 71 ml/100 g/min in the brain stem. Immature rat RCBF, as a proportion of adult rat RCBF, was greatest in brain stem. Previously thought not feasible, this technique provides a reliable and relatively simple means of measuring both CBF and RCBF in the very small laboratory animal.