1. The changes of the ion content, the membrane potential and of the membrane permeability of taenia coli cells have been studied during exposure to K-free solutions. The relative value of the total membrane conductance was determined by measuring the electrotonic potential during constant current pulses with an intracellular electrode. The PK values were calculated from 42K-efflux in K-free solutions.2. In solutions containing penetrating anions the cells initially depolarize. Thereafter they hyperpolarize to about - 85 mV and again depolarize after 90 min to - 5 mV. These potential changes are much smaller if large anions are used as chloride substitutes. Moreover, the final depolarization is only reached after 4-5 hr. This hyperpolarization is not inhibited by 10-5 M ouabain.3. These potential changes are accompanied by a progressive exchange of intracellular K by Na. In solutions containing chloride or nitrate the relative value of the total membrane conductance increases to a maximal value, corresponding to the peak value of the calculated PK. Such changes of the membrane conductance and of PK do not occur in K-free solutions containing large anions.4. It is proposed that the initial depolarization is probably caused by an inhibition of an electrogenic Na pump. In chloride or nitrate solution the hyperpolarization is due to an increase of the [K]i/[K]o ratio and to an increase of the K permeability. In the presence of large anions the hyperpolarization remains small because this increase of PK does not occur.
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