A role in the control of excitability has been attributed to insulin via modulation of potassium (K+) currents. To investigate insulin modulatory effects on voltage-activated potassium currents in a neuronal cell line with origin in the sympathetic system, we performed whole-cell voltage-clamp recordings in differentiated N1E-115 neuroblastoma cells. Two main voltage-activated K+ currents were identified: (a) a relatively fast inactivating current (Ifast − time constant 50–300 ms); (b) a slow delayed rectifying K+ current (Islow − time constant 1–4 s). The kinetics of inactivation of Ifast, rather than Islow, showed clear voltage dependence. Ifast and Islow exhibited different activation and inactivation dependence for voltage, and have different but nevertheless high sensitivities to tetraethylammonium, 4-aminopyridine and quinidine. In differentiated cells − rather than in non-differentiated cells − application of up to 300 nm insulin reduced Islow only (IC50 = 6.7 nm), whereas at higher concentrations Ifast was also affected (IC50 = 7.7 µm). The insulin inhibitory effect is not due to a change in the activation or inactivation current–voltage profiles, and the time-dependent inactivation is also not altered; this is not likely to be a result of activation of the insulin-growth-factor-1 (IGF1) receptors, as application of IGF1 did not result in significant current alteration. Results suggest that the current sensitive to low concentrations of insulin is mediated by erg-like channels. Similar observations concerning the insulin inhibitory effect on slow voltage-activated K+ currents were also made in isolated rat hippocampal pyramidal neurons, suggesting a widespread neuromodulator role of insulin on K+ channels.