We examine the mechanisms underlying the transport of ions across ClC-0 and ClC-1, the two best characterized members of the ubiquitous ClC chloride channel family. Using molecular and Brownian dynamics techniques we create an open channel structure, deduce the permeation characteristics, and highlight the amino acid residues responsible for the different conductance properties of these channels. (C) 2003 Elsevier B.V. All rights reserved.