Giant axons of crayfish were excitable in sodium-free external media containing hydrazinium, hydroxylamine, or guanidinium ions. Action potentials in these solutions were slightly smaller in size and a little longer in duration than those in standard Harreveld's solution. The resting potential was not affected in hydrazine or in hydroxylamine saline, but decreased in guanidine saline. The membrane potential at the peak of the spike (V) was plotted against the univalent cation concentrations in the external medium. The relationship could be expressed by an equation: V=V₀+(RT/F) ln ([Na]+α[C]), where V₀ and α are constants and [Na] and [C] are concentrations of sodium ions and the tested cations, respectively. The parameter α was considered as an indicator to express the ability of substitution of the cation for sodium ion. The order of the ability of the substitution was: Hydroxylamine>Hydrazinium>Guanidinium>Choline.