This paper describes dissipative C1-transport in "porous" lipid bilayer membranes, i.e., cholesterol-containing membranes exposed to 1-3 X 10 -7 M amphotericin B. PDC, (cm. s-), the diffusional permeability coefficient for C-, estimated from unidirectional 86Cl-fluxes at zero volume flow, varied linearly with the membrane conductance (Gm, -1' cm-2 ) when the contributions of unstirred layers to the resistance to tracer diffusion were relatively small with respect to the membranes; in 0.05 M NaCI, PDoI was 1.36 X 10-4 cm s -1 when Gm was 0.02 -1. cm-2. Net chloride fluxes were measured either in the presence of imposed concentration gradients or electrical potential differences. Under both sets of conditions: the values of PDCI computed from zero volume flow experiments described net chloride fluxes; the net chloride fluxes accounted for 90-95 % of the membrane current density; and, the chloride flux ratio conformed to the Ussing independence relationship. Thus, it is likely that C1-traversed aqueous pores in these anion-permselective membranes via a simple diffusion process. The zero current membrane potentials measured when the aqueous phases contained asymmetrical NaCl solutions could be expressed in terms of the Goldman-Hodgkin-Katz constant field equation, assuming that the PDN/PDC1 ratio was 0.05. In symmetrical salt solutions, the current-voltage properties of these membranes were linear; in asymmetrical NaCI solutions, the membranes exhibited electrical rectification consistent with constant-field theory. It seems likely that the space charge density in these porous membranes is sufficiently low that the potential gradient within the membranes is approximately linear; and, that the pores are not electrically neutral, presumably because the Debye length within the membrane phase approximates the membrane thickness.