This video looks at the mechanism of action of rehydration salts – those little sachets that you take away on holiday with you ‘just in case’, but which can be lifesavers for patients with diseases such as cholera. The presentation starts with a description of cell membranes, and how the movement of substances such as ions and sugars is controlled. It then goes on to explain the normal uptake of salts, glucose and water by the cells which line the gut, and how these sachets of rehydration salts save lives, in the treatment of illnesses which result in severe diarrhoea and vomiting.