Single Na channel currents were studied in cell-attached patches from guinea-pig ventricular myocytes in the presence of DPI 201-106. In addition to openings with a single channel conductance of 15 pS, two other types of events occur that are characterized by a conductance of 5 and 8 pS. The low conductance state is less sensitive to block by TTX than the higher conductance states. The medium conductance state has a reversal potential which is similar to the high conductance channel. It is argued that the low conductance channel is a second type of Na channel, whereas the medium conductance channel appears to be a low conductance substate of the normal Na channels.