We have considered the access resistance (AR) of a single conducting c
hannel placed in a membrane bathed by an electrolyte. The classical ex
pression for AR is due to Hall, who modeled the electrolyte as an ohmi
c conducting homogeneous medium. This approach is discussed in the pre
sent paper and it is shown that it is not valid in all cases, but depe
nds on the ion concentration in solution and the ratio between solutio
n and channel resistivities. To get a new expression for AR, we have c
ombined the use of one-dimensional Nemst-Planck and Poisson (NPP) equa
tions for the mouth of the channel and three-dimensional NPP equations
for the outside solution. The influence of ion gradients and the chan
nel itself on AR turns out to be considerable in diluted solutions (an
d also in the case of small channels in any solution). This influence
is weaker in concentrated solutions, for which AR is well described by
Hall's expression. (C) 1998 Elsevier Science B.V.