It is well known that skin conductance is dependent on current flow through skin. It is shown that this may be due to an electro-osmotic effect. This can explain why in some cases negative and in other cases positive potentials increase skin conductance. Electro-osmosis implies current rectification, a powerful skin breakdown mechanism with potentials larger than 50V, and that negative d.c. potentials are more dangerous than positive on dry skin. Electro-osmosis may be a source of error in skin ionic permeability studies with electrical parameters.