The interaction of Brevetoxin 3 (Pbtx-3), a sodium channel activator, with the cardiac sodium channel was studied at the single channel level. It was found that Pbtx-3 (20 microM) shifted steady-state activation to negative potentials, without major effects on the time course of macroscopic activation or macroscopic currents decay, as calculated from averaged single-channel records. Single-channel open times were found to be prolonged. Under the influence of the toxin, sodium channel openings could be observed frequently even at maintained depolarisation. These openings occurred to at least nine different subconductance levels of the open state with smaller conductivities than the maximal one and differed in their open times. Current amplitudes of these open substates were found to cluster around certain amplitude values. Appearance of substates at maintained depolarisation was dependent on the transmembrane potential (Em): Substates with smaller conductivity appeared more frequently at lower Em values whereas at higher Em values substates with higher conductivity values dominated. Furthermore, it was demonstrated that appearance of substates did not result from incomplete recovery from inactivation. From these observations it was concluded that the open substates observed correspond to different conformational states of the channel's activation gates. Under physiological conditions, when the sodium channel opens directly from its closed state these 'incomplete'-open states of the cardiac sodium channel are obscured by fast gating transitions between the corresponding, electrically silent, preopen states. Thus, Pbtx-3 acts mainly via stabilisation of the channel's preopen and different open states. A classification of sodium channel modifiers, based on their interaction with different conformational states of the channel is suggested.