The epithelial cells of the choroid plexuses secrete cerebrospinal fluid (CSF), by a process which involves the transport of Na⁺, Cl⁻ and HCO₃⁻ from the blood to the ventricles of the brain. The unidirectional transport of ions is achieved due to the polarity of the epithelium, i.e. the ion transport proteins in the blood-facing (basolateral) membrane are different to those in the ventricular (apical) membrane. The movement of ions creates an osmotic gradient which drives the secretion of H₂O. A variety of methods (e.g. isotope flux studies, electrophysiological, RT-PCR, in situ hybridization and immunocytochemistry) have been used to determine the expression of ion transporters and channels in the choroid plexus epithelium. Most of these transporters have now been localized to specific membranes. For example, Na⁺-K⁺ATPase, K⁺ channels and Na⁺-2Cl⁻-K⁺ cotransporters are expressed in the apical membrane. By contrast the basolateral membrane contains Cl⁻- HCO₃ exchangers, a variety of Na⁺ coupled HCO₃⁻ transporters and K⁺-Cl⁻ cotransporters. Aquaporin 1 mediates water transport at the apical membrane, but the route across the basolateral membrane is unknown. A model of CSF secretion by the mammalian choroid plexus is proposed which accommodates these proteins. The model also explains the mechanisms by which K⁺ is transported from the CSF to the blood.