1. The fetal brain develops within its own environment, which is protected from free exchange of most molecules among its extracellular fluid, blood plasma, and cerebrospinal fluid (CSF) by a set of mechanisms described collectively as "brain barriers." 2. There are high concentrations of proteins in fetal CSF, which are due not to immaturity of the blood-CSF barrier (tight junctions between the epithelial cells of the choroid plexus), but to a specialized transcellular mechanism that specifically transfers some proteins across choroid plexus epithelial cells in the immature brain. 3. The proteins in CSF are excluded from the extracellular fluid of the immature brain by the presence of barriers at the CSF-brain interfaces on the inner and outer surfaces. These barriers are not present in the adult. 4. Some plasma proteins are present within the cells of the developing brain. Their presence may be explained by a combination of specific uptake from the CSF and synthesis in situ. 5. Information about the composition of the CSF (electrolytes as well as proteins) in the developing brain is of importance for the culture conditions used for experiments with fetal brain tissue in vitro, as neurons in the developing brain are exposed to relatively high concentrations of proteins only when they have cell surface membrane contact with CSF. 6. The developmental importance of high protein concentrations in CSF of the immature brain is not understood but may be involved in providing the physical force (colloid osmotic pressure) for expansion of the cerebral ventricles during brain development, as well as possibly having nutritive and specific cell development functions.