The present study examined temporal changes in activity of large conductance, Ca2+-activated potassium (BKca) channels in postischemic CA1 pyramidal neurons at 2, 6, 24 and 48h after reperfusion. These changes in activity and possible cellular mechanisms were examined using the inside-out configuration of patch clamp. The unitary conductance of postischemic BKca channels increased transiently to 1.9% of the control at 2h after reperfusion, and recovered to the control level thereafter. A persistent increase in [Ca2+]i sensitivity of BKca channels was observed in postischemic CA1 neurons with the maximal sensitivity to [Ca2+]i at 6h after reperfusion while channel voltage- dependence showed no obvious changes. Kinetic analyses showed that the postischemic enhancement of BKca channel activity was due to longer open times and shorter closed times as there was no significant changes in opening frequency after ischemia. Glutathione disulphide markedly increased BKca channel activity in normal CA1 neurons, while reducing glutathione caused a decrease in BKca channel activity by reducing the sensitivity of this channel to [Ca2+]i in postischemic CA1 neurons. Similar modulatory effects on postischemic BKca channels were also observed with another redox couple, DTNB and DTT, suggesting an oxidation modulation of BKca channel function after ischemia. The present results indicate that a persistent enhancement in activity of BKca channels, probably via oxidation of channels, in postischemic CA1 pyramidal neurons may account for the decrease in neuronal excitability and increase in fAHP after ischemia. The ischemia-induced augmentation in BKca channel activity may be also associated with the postischemic neuronal injury.