We study the properties of molten sodium chloride in alternating electric fields of two amplitudes and for a large range of frequencies using nonequilibrium molecular dynamics simulations, and compare the responses with two different methods of temperature control to the predictions of linear response theory. We find that the considerable nonlinearity in the resulting current density observed at low frequencies can be explained by the characteristics of the nonlinear response to constant fields. We also comment on the differences in the dissipation mechanisms and the entropy change with two thermostats. (C) 2003 American Institute of Physics.