The skin is the largest human organ playing an important role in protection, thermoregulation and sensation. Recent studies suggest that a new function has to be added: the storage of sodium. There is increasing evidence that sodium can accumulate in the skin, which suggests that the skin contributes to the regulation of sodium balance in humans, and possibly to the control of extracellular volume and blood pressure homeostasis. The main product of the skin is sweat. Body sweat contains electrolytes and urea. Their concentration can increase considerably when sweat production is stimulated by saunas or hot baths. This finding has motivated studies investigating the effect of stimulated sweating on volume control in patients suffering from kidney disease or heart failure. The physiological concept that sees the skin as third kidney and its possible clinical applications are discussed in this article.