The distributions of transthyretin and albumin in the choroid plexus during brain development have been compared. The South American opossum was chosen because the young are born around the time of choroid plexus formation. Previous work showed that in the adult opossum, transthyretin is expressed in the choroid plexus cells. However, systematic studies of transthyretin in the choroid plexus during development have not been carried out before. Transthyretin was present in 90-95% of the choroidal cells form birth to adulthood. In most cells, transthyretin immunoreactivity was concentrated in the apical region of the cytoplasm. Double labelling of choroid plexus sections with antibodies to albumin and transthyretin showed that 1-2% of cells were positive for both proteins. These findings suggest that from the very earliest stage of choroid plexus formation most epithelial cells both synthesize and contain transthyretin, and a few of these transthyretin-synthesizing cells also contain albumin that is probably being transferred form blood to the cerebrospinal fluid.