Brain water transport is poorly understood at the molecular level, and marked changes occur during brain development. As the aquaporin-4 (AQP4) water channel protein is abundant in brain, the expression levels and subcellular distribution of this protein were examined during postnatal development. This study focused on the cerebellum, which showed the same pattern of AQP4 development as the rest of the brain. Semiquantitative immunoblotting revealed very low levels of AQP4 in the first postnatal week. A pronounced increase was noted in the second week, from 2% of adult level at postnatal day 7 (PN7) to 25% at PN14. At PN1 and PN3 immunofluorescence microscopy revealed weak labelling, mainly in radial processes (Bergmann fibres) and at the pial surface. Between PN7 and PN14 the labelling underneath the pia showed a strong increase, and immunoreactivity also appeared around blood vessels throughout the cerebellum. High-resolution immunogold electron microscopy revealed that the subpial and perivascular labelling was restricted to glial end feet, notably to those plasma membrane domains that were apposed to the basal laminae. At no stage was there any evidence of neuronal AQP4 labelling, and AQP1, -2, -3 and -5 proteins were not detected in the neuropil. Riboprobes to AQP4 mRNA produced a particularly strong in situ hybridization signal in glial cells between PN7 and PN14, corresponding to the stage of the most rapid increase of AQP4 protein. The time course and pattern of AQP4 expression suggests that this aquaporin plays an important role in brain water and K+ homeostasis from the second week of development.