1. Whole-cell recordings were made from rat CA1 neurones in brain slices. When electrodes contained diazo-2 (2 mM) or dibromo BAPTA (1 mM) a large steady-state outward current (hundreds of picoamps) developed within 5 min of breakthrough at a VH of -60 mV. BAPTA itself (1 mM) caused qualitatively similar but smaller effects. 2. The outward current was accompanied by increased conductance with a null potential close to the calculated K+ equilibrium potential (EK) of -110 mV. Development of outward current occurred concurrently with progressive loss of slow AHP tail current (IsAHP) evoked by brief depolarizations. The peak latency of IsAHP increased during the onset of chelator action. 3. The persistent outward current was reversibly inhibited by noradrenaline (10 microM) or isoprenaline (2-5 microM), and completely prevented by 8-bromoadenosine 3',5' cyclic monophosphate (8-Br cAMP; 100 microM) or QX-314 (10 mM) in recording electrodes. After development of outward current, diazo-2 photolysis caused inward current and decreased conductance. Both flash- and noradrenergic-sensitive responses were inwardly rectifying outward currents with null potentials close to EK. 4. The outward current induced by dibromo BAPTA was not blocked by internal EGTA (10 mM). However, experiments incorporating Ca2+ influx or Ca2+ loading of the buffer indicate that Ca2+ facilitated the outward current. 5. The outward currents induced by dibromo BAPTA or diazo-2 were not associated with significant changes in resting [Ca2+]i. Regions of the cell contributing to the outward current were deduced from measurements of fura-2 diffusion. These were compared with regions of [Ca2+]i elevation during IsAHP. 6. These results are consistent with the hypothesis that the BAPTA series Ca2+ buffers can activate those Ca2+-activated K+ channels that underlie the slow AHP, without the predicted elevation of bulk [Ca2+]i. Therefore these results cannot be interpreted solely in terms of Ca2+ concentration changes, although the observations illustrate a novel, investigative role for these compounds in the study of Ca2+-dependent processes.