The cinchona alkaloids quinine and quinidine have been shown to block a broad range of voltage-gated membrane conductances in a variety of excitable tissues. Using the whole-cell version of the patch clamp technique, we examined the effects of these compounds on voltage-dependent currents from horizontal cells dissociated enzymatically from the all-rod retina of the skate. We report here a novel and unexpected action of quinine and quinidine on isolated horizontal cells. In addition to blocking several of the voltage-activated currents of these cells, the introduction of the alkaloids evoked a large outward current when the cells were held at depolarized potentials. Using tail current analysis, the reversal potential of the outward current was close to O mV, and the current was markedly suppressed by extracellularly applied cobalt, acetate, and halothane. Depolarization in the presence of quinine also permitted entry into the cells of extracellularly applied Lucifer yellow (MW = 443 D), whereas a 3-kD fluorescein-dextran complex was excluded. These findings suggest that the large, apparently nonselective conductance induced by quinine and quinidine results from the opening of hemi-gap junctional channels.