1. The transfer of albumin between the blood and the cerebrospinal fluid (CSF) has been investigated in neonatal (3 days old) and juvenile (20 days old) rats. At both stages of postnatal development, all of the albumin present in the CSF can be accounted for by transfer from the blood. Thus it is unlikely that in situ synthesis of albumin contributes to the naturally high levels of albumin in CSF in the developing brain. 2. The high concentration of albumin in CSF of the neonatal rat brain cannot be accounted for solely by diffusion from the blood. In the 3-day-old rat, only about one quarter of the albumin in CSF enters by diffusion from the blood, whilst the remainder appears to be transported into the CSF by a specific mechanism which can discriminate between different species of albumin. The specific transport component of albumin transfer between the blood and the CSF appears to be developmentally regulated and is not apparent in 20-day-old rats. 3. Chemical modification of albumin resulting in either an increase or a decrease in electrophoretic mobility (at pH 7.4), significantly reduces blood-CSF transfer of albumin in 3-day-old rats, but has little effect in the 20-day-old rat. Thus overall molecular charge does not appear to be an important feature of the species-specific blood-CSF albumin transport mechanism in neonatal rats.