1. Voltage-activated K+ currents were recorded from visually identified inhibitory interneurones of the CA1 stratum oriens-alveus region in neonatal rat hippocampal slices using outside-out patch and whole-cell voltage clamp techniques. 2. Outward currents comprised both a transient and a sustained component when elicited from a holding potential of -90 mV. Tail current analysis of current reversal potentials showed that outward currents were carried by potassium ions. 3. The transient current, IA, was activated with a time to peak within 5 ms, inactivated with a time constant of approximately 15 ms at 0 mV and possessed half-activation at -14 mV. Half-inactivation of the transient current occurred at -71 mV. At -90 mV, the transient current recovered from inactivation with a time constant of 142 ms. 4. Activation of currents from a holding potential of -50 mV permitted isolation of the sustained current, IK. In Ca(2+)-free conditions the sustained current showed rapid activation, reaching about 80% of its maximum within 1.5 ms, and showed little inactivation during 1 s depolarizing steps. The majority of sustained outward currents showed no voltage-dependent inactivation. In approximately 20% of cells, a slow time-dependent inactivation of the sustained current was observed, suggesting the presence of a second type of sustained current in these cells. 5. A Ca(2+)-dependent K+ current comprised a significant portion of the total sustained current; this current was activated at voltages positive to -30 mV and showed no time-dependent inactivation over a 1 s depolarizing step. This current component was removed in Ca(2+)-free conditions or by iberiotoxin. 6. Low concentrations of 4-AP (50 microM) attenuated both the transient and sustained current components recorded in a Ca(2+)-free solution. Higher concentrations of 4-AP (< 10 mM) were without further effect on the sustained current but completely blocked the transient current with an IC50 of 1.8 mM. TEA blocked the sustained current with an IC50 of 7.9 mM without significantly reducing the transient current. Both current components were resistant to dendrotoxin (500 nM).