The issue of membrane capacitance (C) is re-examined. The influence of fluctuations is studied using the non-local electroelastic model developed previously. The effect of undulations is compared with that due to voltage-induced thinning of the bilayer. It is shown that the undulatory contribution to the voltage dependence ofCis significant at small surface tensions, but at higher tensions the role of compression becomes dominant. At all tensions, the effect of non-locality does not exceed 15–20% of the total undulatory contribution. The same results hold if ionic screening by the electrolyte (in a Poisson-Boltzmann approximation) is taken into consideration.