The study gives an overview on the regulation of cerebral water content and of brain volume. The molecular mechanisms of the development and resolution of various oedema forms are discussed in detail. The physiological and pathophysiological role of the recently discovered molecular water channel proteins aquaporin-1 (AQP1) and aquaporin-4 (AQP4) as well as the importance of central neuroendocrine regulation by vasopressin and atriopeptin are reviewed based on the relevant literature and personal studies. Quantitative water maps based on the combination of multicompartment-T2, diffusion weighted MRI and T1 studies have proven to be powerful tools for studying new drugs against brain oedema brought about by various neuropathological conditions and for testing their efficacy both in animal experimental and clinical conditions. Non-peptide vasopressin antagonists, atriopeptin agonists and drugs targeting AQP4 are potential new families of oedema-decreasing drugs.