1. The intracellular Cl concentration of taenia coli cells, determined by an analytical procedure and by an extrapolation procedure, has a value between 60 and 73 m-mole/l. cell water.2. This concentration is too high to be explained by a passive distribution. The discrepancy could be due to a binding of Cl in the intracellular or extracellular compartment or to an active uptake of Cl by the cells.3. Determination of the activity coefficient for Cl in homogenates of smooth muscle did not support the hypothesis of binding of Cl ions.4. The efflux of 36Cl from taenia coli cells was not affected by foreign anions. After 1 hr exposure to a Cl-free solution, the tissues contained less than 1 m-mole of Cl/kg wet wt., even if Cl had been replaced by a slowly penetrating anion. Because the intracellular cation concentration remained constant, it has to be assumed that new anionic groups can be formed in the cells.5. The intracellular Cl concentration decreases during exposure to ouabain or to K-free solution. The uptake seems therefore to be linked to the uptake of K through the Na pump.6. Exposure to K-free solution increases the K permeability of the membrane. Under the same experimental conditions the Cl permeability of the membrane increases as long as K is leaking out of the cells.7. The anions in the external solution exert an important influence on the K permeability of the membrane. NO3 and I cause a small increase of the permeability and large anions such as benzenesulphonate, propionate or pyroglutamate cause a pronounced decrease of this permeability.
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