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1. The long-term effects of ouabain on the membrane potential of the Anisodoris giant neurone (G cell) were examined in cells maintained for periods of up to 15 hr at 11-13° C.

2. In the presence of ouabain (5 x 10^-4 M), the membrane potential depolarized to a constant level for 1-4 hr, then hyperpolarized for 5-7 hr after which it gradually depolarized again.

3. During the hyperpolarizing phase, after 6-8 hr in ouabain, [K]₁ fell approximately 50%, [Na]₁ increased 50-100% and the P_Na/P_K ratio decreased to 25% of its initial value.

4. After 8 hr in ouabain the membrane conductance increased two- to fourfold. This increase was independent of temperature and membrane rectification.

5. The K permeability (P_K) was calculated from the constant field equation, and showed a fourfold increase after long-term treatment with ouabain. This rise in P_K probably underlies the membrane hyperpolarization and the decrease in the P_Na/P_K ratio.

6. It is suggested that inhibition of the Na⁺ pump with ouabain causes a gradual rise in [Na]₁ which secondarily leads to Ca²⁺ uptake, an increase in [Ca]₁, and thereby an increase in P_K.