• 1)
    It is shown that for a free ion diffusing through a membrane the ratio between the flux in one direction and the simultaneous flux in the opposite direction is independent of the structure of the membrane. This ratio inline image, which can be evaluated through tracer experiments, is equal to the ratio inline image between the electrochemical activities of the ion on the two sides of the membrane.
  • 2)
    Deviations from the equation inline image indicate that the ion does not diffuse in the free state only, but, in part at least, as a component of some other moving particle in the membrane. Complex formation for instance as a part of an active transport mechanism brings about deviations from the above equation.
  • 3)
    The diffusion of iodide through the isolated surviving frog skin is studied by means of the radioactive I131. I- diffuses inwards faster than outwards under all conditions studied. Nevertheless, no active transport of I- need be postulated since the potential difference across the skin is somewhat higher than required to explain the difference in diffusion rate.
  • 4)
    Both influx and outflux of I- show a negative correlation to the potential difference, so that high P. D. values are found only when the I- permeability is low.