The membrane current density, I(m), in the squid giant axon has been calculated from the measured external current applied to the axon, I(o), by the equation See PDF for Equation where V(m) is the membrane potential under the current electrode and r(1) and r(2) are the external and internal longitudinal resistances. The original derivation of this equation included in one step an assumption of a linear relation between I(m) and V(m). It is shown that the same equation can be obtained without this restricting assumption.