The nonlinear optical properties of dyes that alter their optical characteristics rapidly with membrane potential are described. The second harmonic signals from these dyes characterized in this paper are among the largest that have been detected to date. Structural conclusions are drawn from the second harmonic signals generated by the Langmuir Blodgett monolayers used in these measurements. Our results indicate that with appropriate instrumentation second harmonic signals could readily be detected from living cells stained with these dyes.