To examine the quantitative relationship between solution specific conductivity and the permeability of tetracaine HCl, and to investigate the effect of receptor solution specific conductivity on the iontophoretic transport.An in vitro study was carried out to determine the iontophoretic permeability of tetracaine hydrochloride through rat skin. Iontophoretic flux of tetracaine hydrochloride through excised rat skin was determined using Valia-Chien two-chamber diffusion cells with a constant d.c. current and Ag/AgCl electrodes. The specific conductivities of donor and receptor solution were also measured.Iontophoretic flux of tetracaine hydrochloride increased with a decrease of anion (chloride ion) concentration in receptor. And the iontophoretic permeability (ER, ER is the enhancement ratio, and ER = iontophoretic flux/passive flux) for tetracaine hydrochloride was directly related to the conductivity of receptor solution when other conditions were held constant. Linear regressions confirmed that ER was related to inverse of overall specific conductivity of donor and receptor solution [1/(ks.d + ks.r), ks.d and ks.r are the specific conductivity of donor and receptor solution].The results suggest that specific conductivity of receptor solution may be a important factor for the iontophoretic permeability of a solute.