Inherited disorders of ion‐channels are associated with paroxysmal dysfunction of excitable tissues and manifest as diseases of the brain, heart and skeletal muscle. These so‐called channelopathies have now been described for most of the major categories of voltage‐dependent ion‐channels including those selectively permeable to sodium. Sodium channelopathies affecting the heart and brain are reviewed in this essay. They show striking differences and similarities including, for example, their responsiveness to changes in body temperature and sleep state. They represent a paradigm for (...) efforts to trace disturbed behaviour of physiological systems back to its molecular origins and understanding their molecular basis may provide clues to important health issues such as cardiac side effects of drugs and response to medication used to treat epilepsy. BioEssays 25:981–993, 2003. © 2003 Wiley Periodicals, Inc. (shrink)