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1. Tracer movements and ionic composition have been observed in the smooth muscle of the guinea-pig taenia coli in vitro at 4, and 35° C. The muscle was quiescent at 4° C, and did not show spontaneous electrical nor mechanical activity for 25 min after re-warming. It was therefore possible to make observations on the quiescent tissue at 35° C.

2. The rate of loss of tracer sodium was sensitive to changes of temperature, with an average Q₁₀ of 3·4 ± 0·4 on warming in the range 4-35° C. Exponential analysis of ²⁴Na efflux at 35° C showed an initial rapid phase, a second phase with t = 2 min, and a slowly exchanging phase. The second phase was most sensitive to changes of temperature and contained 15 m-mole Na/kg fresh wt. The corresponding transmembrane flux in spontaneously active muscle at 35° C would then be 30 p-mole cm^-2 sec^-1, and 20% of the available metabolic energy would be needed to maintain this exchange.

3. The rate of loss of tracer sodium was reduced in potassium-free or sodium-free solutions, but the effects were not impressive.

4. The cell size increased at 4° C, and fell again on warming while Na and Cl left the cell and K was taken up. Cell chloride content was linearly related to cell size.

5. The observations are interpreted on the assumption that the cytoplasm is osmotically isotonic and electrically neutral, and that some Na, K and Cl are sequestered elsewhere in the tissue. It is concluded that the quiescent smooth-muscle cell membrane is rather more permeable to sodium than the skeletal-muscle cell membrane.