The depolarising drug, decamethonium, is a permeant cation that binds in the junctional region, accumulating steadily allowing the influx to be followed (Creese and England, 1970). Following exposure to labelled cation and wash in inactive solution, the uptake of permeant cations can be expressed initially as (radioactivity mg^-1/(radioactivity pl^-1 solution), which is pl mg^-1, a clearance. This multiplied by the external concentration gives the uptake as pmol mg^-1. In cultured muscle cells, the uptake of sodium in the presence of carbachol has been interpreted in terms of a single-ion water-filled pore by Huang et al (1978). In mammalian muscle, sodium influx by tracer methods has been complicated by the sodium pump. By slicing frozen diaphragm muscle, one can separate the junctional uptake from the non-junctional (Creese et al 2003). In the model of Huang et al (1978), the diffusion equation can be applied, such that the influx of permeant cations like sodium is proportional to the diffusion coefficient (D) X external concentration [Na_o] less a smaller factor given by D X [Na_i] the internal sodium. Efflux of junctional sodium is completely suppressed by ouabain (Creese at al 2003), so allowing further simplification of the single-ion water-filled pore model, with the sodium influx proportional to the diffusion coefficient (D) multiplied by the external sodium concentration (145 mM). Then D is proportional to flux/concentration, which is a clearance rate (pl mg^-1 s^-1), and with two cations the ratio of diffusion coefficients (R, decamethonium/sodium) would equal the ratio of clearance rates. Then sodium influx could be estimated by the accumulation of decamethonium, plus knowledge of the diffusion coefficients. From the work of Brooks and Mackay (1971) the equivalent conductance ratio for decamethonium/sodium is near 0.89, and this could be used for the ratio of diffusion coefficients. With guinea pig diaphragm, the junctional influx of decamethonium was 0.320μl mg^-1 hr^-1 and with 130 fibres mg^-1, (Creese et al 1987), the estimate of sodium influx, with the faraday as 0.965 x 10⁵ coulomb equiv ^-1, equals 9.6 nA X (1/R). This compares with endplate current of 11nA obtained by electrical recording on withdrawal of depolarising drug (Creese et al., 1987).