1. In the guinea‐pig taenia coli, influences of Na and Ca ions on the recovery process from the K contracture were investigated. In the absence of Na ion (sucrose‐Krebs solution), the K contracture did not recover when the external K (143 m M) was returned to the normal concentration (5·9 m M), although the membrane was repolarized to normal resting potential.2. After reducing the external K concentration to normal, the addition of Na rapidly terminated the contracture. About 5 m M‐Na was enough to produce the relaxation, but the rate of relaxation was slower the lower the Na concentration.3. Lithium could substitute for Na in the relaxation, but Tris‐hydroxymethyl aminomethane could not. The possibility of a chloride contribution was excluded.4. Ouabain (2 × 10−6 g/ml.) and K removal reduced the rate of relaxation by Na ion only slightly. Lowering the temperature also had a small effect, having a Q10 of about 1·4. Therefore, the Na‐K pump may not be involved in this process, but a physical process seems responsible.5. The contracture in K‐Krebs solution and in sucrose‐Krebs solution was dependent on the external Ca concentration suggesting a high Ca permeability of the membrane. When sucrose was isosmotically replaced with Mn, Mg, La or Ca ions the relaxation was produced with a relatively fast speed in the absence of external Na ions.6. These results may be explained by assuming that external Na ions are involved in decreasing the Ca permeability of the membrane and in reducing the intracellular Ca concentration by Na‐Ca exchange, energy for which is supplied by Na influx. In the relaxation by polyvalent cations, suppression of the Ca permeability is probably the main factor.