Prolonged depolarization of isolated, voltageclamped skate retinal horizontal cells produces an outward current that exhibits a late onset and develops slowly with time. This current, which we refer to as the Q-current, is associated with an increase in membrane conductance, and is present when other voltage-gated conductances have been pharmacologically blocked. The reversal potential for the Q-current, obtained using tail current analysis, was close to 0 mV. The magnitude of the current was greatly reduced by superfusion with 25 mM acetate, and by 4 mM cobalt chloride, 2 mM l-octanol, and a saturated solution of the general anesthetic halothane. In addition, the low-molecular weight fluorescent dye Lucifer yellow, applied extracellularly, entered the cells during activation of the Q-current, whereas a 3 kD dextran-fluorescein complex did not cross the cell membrane. The effects of divalent cations, the nonspecific nature of the ionic current suggested by its reversal potential, the entry of Lucifer yellow, and the ability of acetate, halothane, cobalt, and octanol to block the current lead us to hypothesize that the Q-current results from the opening of hemi-gap junctional channels that mediate electrical coupling between skate horizontal cells.