Quinidine is a commonly used antiarrhythmic agent and a tool to study ion channels. Here it is reported that quinidine equilibrates within seconds across the Sf9 plasma membrane, blocking the open pore of Shab channels from the intracellular side of the membrane in a voltage-dependent manner with 1:1 stoichiometry. On binding to the channels, quinidine interacts with pore K(+) ions in a mutually destabilizing manner. As a result, when the channels are blocked by quinidine with the cell bathed in an external medium lacking K(+), the Shab conductance G(K) collapses irreversibly, despite the presence of a physiological [K(+)] in the intracellular solution. The quinidine-promoted collapse of Shab G(K) resembles the collapse of Shaker G(K) observed with 0 K(+) solutions on both sides of the membrane: thus the extent of G(K) drop depends on the number of activating pulses applied in the presence of quinidine, but is independent of the pulse duration. Taken together the observations indicate that, as in Shaker, the quinidine-promoted collapse of Shab G(K) occurs during deactivation of the channels, at the end of each activating pulse, with a probability of 0.1 per pulse at 80 mV. It appears that when Shab channels are open, the pore conformation able to conduct is stable in the absence of K(+), but on deactivation this conformation collapses irreversibly.