The basolateral membrane of isolated strial marginal cells has been probed for conductive pathways by the patch-clamp technique. Two types of voltage-insensitive channels were identified in both cell-attached and excised patches. Of these, frequently (69% of excised patches) observed was a Ca 2+-activated nonselective cation channel having a unit conductance of 24.9 ± 0.5 pS ( N = 16). Other characteristics of this type in excised patches include: 1) linear I-V relations with 150 mM K + (pipette)/150 mM Na + (bath), 2) a permeability sequence of NH 4 +> Na + = K + = Rb +>Li +, 3) a flickering block by quinine or quinidine (both 1 mM), and 4) a dose dependent block of its activity by ADP or ATP (IC 50,ATP/IC 50,ADP = 20–35), both from the cytosolic side. Channels with similar characteristics were found in the apical membrane of the same cell; however, the basolateral channels were 2–4 times more densely distributed than the apical counterparts. Also frequently (57%) detected was a Cl − channel of 80.0 ± 0.5 pS ( N = 6), whose activity was Ca 2+ independent. Additionally, this Cl − channel had: 1) linear I-V relations with symmetric Cl −, 2) a permeability sequence of Cl −>Br −>I −≥NO 3 −≥gluconate −, and 3) a complete and reversible block by 1 mM diphenylamine-2-carboxylate. In contrast to the apical Cl − channels, the basolateral ones had a much higher density (57% vs. < 1%) as well as a higher unit conductance (80 pS vs. 50 pS) than the apical counterpart. The relative abundance of these two types as the major conductive pathways for Na +, K +, and Cl − in the basolateral region must be taken into account when addressing the role of strial marginal cells in generating the positive endocochlear potential. The Cl − channel may facilitate Cl − distribution across the basolateral membrane.