A method is described for measuring the amount of sodium confined between an outer membrane, which is passively but specifically permeable to sodium, and an inner membrane where the active transport mechanism is located. The method allows correction for sodium outside the transport pool. Several determinations can be performed on the same skin. Repetitive determinations gave a standard deviation of ± 7 per cent. A single determination can ordinarily be made in less than an hour. Good agreement is found under suitable conditions between the potential difference calculated from the pool and the measured potential difference across the skin. Furthermore, from the disappearance rate of sodium from the pool and the size of the pool the active sodium transport can be calculated under conditions where other methods cannot distinguish between active and passive movements of sodium.