1. A sucrose gap technique was used to study the effect of a sodium ionophore on the potential changes which occur during superfusion with potassium-free solution. Crucial values were checked with a microelectrode technique. 2. Potassium-free solution induces a complex response consisting of a hyperpolarizing phase (HP) and then a depolarizing phase (DP) during exposure to zero K +, followed by a transient extra hyperpolarizing phase (EHP) on readmission of K +. 3. The sodium ionophore, monensin, has the effect of increasing the amplitude of the DP, in perfusion medium of normal Na + content. This is similar to the effect of treatment with neurohumors and to the after-effect of direct electrical stimulation. The effects of ACh and monensin are additive. 4. In low na + medium, monensin has the effect of increasing the amplitude of the HP. 5. These effects are consistent with an action of monensin in increasing Na + flux in the direction of the concentration gradient and support the hypothesis that neurohumors stimulate the sodium pump by increasing Na + influx.