Band 3 is proposed to contain substrate channels that lead from the aqueous medium to a transport site buried within the membrane, and which can be blocked by inhibitors. The inhibitors 1,2-cyclohexanedione (CHD) and dipyridamole (DP) each inhibit the transport site 35Cl NMR line broadening, but neither competes with Cl- for binding. Thus these inhibitors do not occupy the transport site; instead they slow the migration of Cl- between the transport site and the medium. The simplest explanation for this behavior is that CHD and DP block one or more substrate channels. CHD is an arginine-specific covalent modification reagent, and its effectiveness as a channel blocker indicates that the channel contains arginine positive charges to facilitate the migration of anions through the channel. DP is a noncovalent channel blocker that binds with a stoichiometry of 1 molecule per band 3 dimer. DP binding is unaffected by CHD but is prevented by phenylglyoxal (PG), 4,4'-dinitrostilbene-2,2'-disulfonate (DNDS), or niflumic acid. Thus the DP and CHD binding sites are distinct, with DP binding sufficiently close to the transport site to interact with PG and DNDS. It is proposed that substrate channels may be a general feature of transport proteins.