The blood-brain barrier (BBB) to water-soluble drugs and macromolecules can be opened in vivo by infusing a hypertonic solution of arabinose or mannitol into the carotid artery for 30 sec. Opening involves widening of tight junctions between endothelial cells of the cerebrovasculature and is mediated by endothelial cell shrinkage, vascular dilatation associated with removal of water from brain, and modulation of the contractile state of the endothelial cytoskeleton and junctional proteins by increased intracellular calcium. A 10-fold increase in BBB permeability to intravascular substances, lasting about 10 min following osmotic exposure, reflects both increased diffusion and bulk fluid flow from blood into brain. Furthermore, recent evidence indicates that the duration of peak BBB opening can be extended beyond 30 min, by pre-treatment with a Na(+)/Ca(2+) channel blocker. In experimental animals, the osmotic method has been used to grant wide access to brain of water-soluble drugs, peptides, antibodies, boron compounds for neutron capture therapy, viral vectors for gene therapy and enzymes. Ongoing multi-centre clinical studies suggest that the method, when used with intra-arterially administered anticancer drugs, can prolong survival in patients with malignant brain tumours, with minimal morbidity. However, controlled clinical trials are critical to see if the osmotic procedure with intra-arterial drugs enhances survival in brain tumour patients compared with intra-arterial drug alone.