The reported experiments demonstrate that in rabbit gallbladder epithelium: a) the mucosa-cell Na+ and Cl- influxes (45 sec long) are reduced in Cl- -free and Na+ -free bathing solutions respectively; transconcentration effects are observed. b) Cell Cl- labelling through the luminal membrane, with 36Cl- (1 hr incubation), is nearly abolished if a Na+ -free bathing solution is used; but cell Na+ labelling with 22Na+ in the lumen (1 hr incubation) does not change if a Cl- -free bathing solution is employed. c) Cell Na+ and Cl- labelling through the basolateral membrane (1 hr incubation) is negligible; some Cl- labelling is obtained only when the intracellular electrical potential is artificially reduced. d) SO 2 4- is found to abolish net water transport, to cross the epithelium, but not to enter the cell. On the basis of these data a paracellular pathway for net Cl- transport is ruled out, a Na+ and Cl- cotransport through the cell luminal barrier and a Na+ active extrusion through the basolateral membrane are suggested. No definitive conclusion about Cl- extrusion is possible.