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1. The intra- and extracellular spaces and ionic content of the taenia coli of the guinea-pig have been measured in a series of bathing solutions in which the tonicity varied from 0.5 to 3 times the tonicity of the normal Krebs solution.2. Equilibrium of the tissue in the experimental solution is reached within about 30 min.3. The absolute values of the parameters measured have been shown to depend on the blotting technique used. These differences were eliminated by expressing the results as a percentage of the values found in the normal Krebs solution.4. In hypertonic solutions the cell behaves as a perfect osmometer, the cell volume changing in proportion to the tonicity of the bathing medium. Only a small amount of cations is lost from the cells. A considerable amount of chloride is lost, making the postulation of its replacement by some other anions necessary in order to maintain electroneutrality of the intracellular solution.5. In hypotonic solutions the cells do not behave as predicted for a perfect osmometer. In 0.5 hypotonic solution an actual decrease in cell volume was observed associated with an increase of the extracellular space probably due to penetration of [(14)C]sorbitol into the cell. The intracellular ionic concentration was decreased. These findings suggest damage of the cell membrane.6. The observed hyperpolarization of the membrane in hypertonic solution can be explained by the increased intracellular potassium concentration.