The transport pathways for chloride in basolateral membrane vesicles from the rabbit renal cortex were investigated. 36Cl uptake was stimulated by the presence of potassium in the uptake media compared with sodium or N-methyl-D-glucamine. In addition, potassium (86Rb) uptake was stimulated more by chloride than by nitrate or gluconate. Neither of these processes was further stimulated by potassium gradients plus valinomycin, suggesting the presence of an electrically neutral K-Cl cotransport system. A magnesium-induced chloride conductance was also found in the basolateral membrane vesicles. In the absence of magnesium, the chloride conductance was low; valinomycin and an inwardly directed potassium gradient did not stimulate 36Cl uptake, anthracene-9-carboxylic acid did not inhibit 36Cl uptake, and valinomycin did not stimulate chloride-dependent 86Rb uptake. However, in the presence of 1 mM magnesium, opposite results were obtained; valinomycin and an inwardly directed potassium gradient stimulated 36Cl uptake, and valinomycin stimulated chloride-dependent 86Rb uptake. Therefore, an electrically neutral K-Cl cotransport and magnesium-induced chloride conductance were found in renal cortical basolateral membrane vesicles prepared from the rabbit renal cortex.