1. Removal of the external K ions depolarized the membrane of the guinea-pig taenia coli by about 5 mV, and increased the frequency of the spontaneous spike activity. After about 20 min in K-free solution, the frequency gradually decreased and spontaneous activity stopped after about 60 min.2. Readmission of K produced a transient hyperpolarization by 10-15 mV for 10-20 min, and lowered the excitability. The membrane resistance decreased during the hyperpolarization. When the external K concentration was returned to normal before the spontaneous activity had disappeared in K-free solution, there was a complete suppression of the spontaneous electrical and mechanical activities, accompanied by hyperpolarization of the membrane. The duration of this suppression of electrical and mechanical activities depended on the exposure time to K-free solution, the recovery being delayed by longer exposure.3. The effects of removing external K were essentially the same in the presence of tetrodotoxin (10(-7) g/ml.), of atropine (10(-6) g/ml.), and also in low Cl (2-7 mM).4. The effects of ouabain (10(-6) g/ml.) were similar to those of zero K but appeared more slowly. After treatment with ouabain, the membrane was hyperpolarized by removing the external K and there was no hyperpolarization nor suppression of the mechanical activity on readmission of external K.5. Rb substitution for K produced little effect and addition of 5.9 mM-Rb after K-removal produced a typical suppression of activity as the readmission of K. When K was replaced with an equimolar concentration of Cs, the effects were similar to those produced by K-free solution.6. The pattern of the response to a change in the external K concentration was different below 23 degrees C. The spontaneous activity was suppressed by removal of K, and increased by returning the external K.7. It is concluded that, in the taenia, the membrane potential can be modified by an electrogenic pump, the activity of which is inhibited by removing K, by ouabain, and by lowering the temperature to about 23 degrees C. During the recovery from the exposure to K-free solution or to ouabain, the pump activity is potentiated, causing hyperpolarization of the membrane by 10-20 mV. Rb can, Cs cannot, replace K in activating the pump at the same concentration as K.