Voltage-gated Na+ channels (VGSCs) initiate and conduct action potentials in excitable cells. Functional VGSCs are also present in cells from a number of cancers. Inhibiting Na+ current (INa+) reduces cancer cell migration and invasiveness (Brackenbury, 2012). We previously reported that phenytoin, an antiarrhythmic agent, inhibits INa+ in human MDA-MB-231 breast cancer cells, thus reducing migration and invasion (Yang et al., 2012). Membrane potential (Vm) is finely tuned in cellular activities such as proliferation and differentiation. Cancer cells possess depolarised Vm (Marino et al., 1994), which favours cell cycle progression (Sundelacruz et al., 2009). Here, we used whole-cell current clamp to study the involvement of VGSCs in regulating Vm of MDA-MB-231 cells. Cells were firstly perfused with standard physiological saline solution (PSS) for 60s, followed by a ‘test’ PSS for 150s, and finally a wash-out step with standard PSS for a further 150s. Vm values (sampled at 100Hz) within the last 5, 15, 30 and 60s in each of the 3 stages were analysed using ANOVA followed by Tukey post-hoc tests. Values are mean ± SEM. We firstly replaced extracellular Na+ with choline in the PSS. Na+ depletion reversibly hyperpolarised the Vm by ~10mV (Table 1). We next tested if VGSCs are involved in Vm regulation by applying 10 μM phenytoin in the PSS. Phenytoin hyperpolarised the Vm by ~3mV while the drug carrier 75μM NaOH had no effect (P=0.98). The effect of phenytoin was thus more subtle than Na+ depletion. Interestingly, after washing out phenytoin, the Vm did not recover (Table 2). We conclude that (1) Na+ conductance contributes to the depolarised Vm in MDA-MB-231 cells, and (2) phenytoin partially inhibits the Na+-mediated Vm depolarisation in a non-reversible manner. Given that depolarised Vm is a hallmark of malignancy (Sundelacruz et al., 2009), we propose that Na+-mediated Vm regulation may play a role in cancer progression.