The purpose of the present study has been to work out a method for the determination of the rate of exchange of Na-and C1-ions between the fibres of isolated frog sartorius and the surrounding medium.From HEPPEL'S work on rats (1939) it is known that intrafibrillar Na exchanges a t a fairly high rate, an approximate equilibrium between fibre Naa4 and serum Naa4 being obtained within one hour. The determination of the rate of exchange was possible only on rats with increased Na concentration in the fibres, a condition brought about by feeding the animals a K-free diet.The method used in the present paper should make possible the measurement of the rate of exchange even when the intracellular Na represents only a small fraction of the total Na content of the muscle.The principle applied is to soak the muscle in a solution containing Naa4 until a reasonable concentration of the isotope within the fibres is reached. Then, the washing out of the active ions with Naa*-free Ringer is followed over a period of time and from the rate of disappearance of activity from the muscle the diffusion rate of Na in the interspaces as well as the renewal rate of fibre Na can be estimated (see below).The peculiar ion distribution between the muscle fibres and the surrounding solution makes imperative the assumption of a n active extrusion of Na+ from the fibres (DEAN 1941, KROGH 1946, CONWAY 1947). Based partly on the experiments t o be de-

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