The proteins secreted by the choroid plexus throughout rat brain development were analyzed by two-dimensional polyacrylamide gel electrophoresis following biosynthetic labeling of choroid plexus pieces with [14C]leucine in vitro. Approximately 20 major protein species were resolved which, with the exception of transferrin, transthyretin, and alpha 2-macroglobulin, appear to be unrelated to proteins found in high concentrations in plasma. Several patterns of developmental regulation were observed. At least two of the proteins were synthesized and secreted at high levels only by fetal choroid plexus, whereas the secretion of several other proteins including transferrin and proteins comigrating with cystatin C and alpha 2-macroglobulin increased only after birth. The levels of mRNA coding for transferrin, ceruloplasmin, cystatin C, alpha 2-macroglobulin, beta 2-microglobulin, and transthyretin were measured in the brain during development by dot hybridization and northern gel analysis. No mRNA was detected coding for the proteins alpha-fetoprotein, alpha 1-antitrypsin, haptoglobin, and thiostatin in the brain at any stage. For those proteins, which are produced in other parts of the brain as well as by the choroid plexus, the changes in their corresponding mRNA levels measured in whole brain paralleled the changes in their secretion by the choroid plexus. The results presented in this paper show that the choroid plexus is active in protein secretion at all stages studied. The changing pattern of protein secretion by the choroid plexus, combined with its early development compared with other tissues in the brain, suggests that it is active in providing the appropriate extracellular environment for the growth and differentiation of the brain.