Fresh, empty, isolated sacs of leg skin from R. pipiens manufacture a salt solution reasonably balanced with respect to ions and approximating a somewhat dilute Ringer's fluid. Concentrations of ions bear little relationship to concentrations in the external medium even down to 12 mM. An effective regulatory mechanism is indicated whereby the amount of salt transported is adjusted to the amount of water or vice versa, the rate of movement of either salt or water being largely independent of osmotic or ionic gradients (outer fluid to manufactured inner fluid). Concentrations of the inner fluids appear to be regulated to conform to a fairly constant concentration within the skin. Some evidence is presented that a major factor in regulating concentration of the inner fluid is triggered by an initial dilution of the inner fluid, followed by stimulation of salt uptake.
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