1. Two transient outward currents were identified in large pyramidal neurones from layer V of cat sensorimotor cortex ('Betz cells') using an in vitro brain slice preparation and single-microelectrode voltage clamp. Properties of the currents deduced from voltage-clamp measurements were reflected in neuronal responses during constant current stimulation. 2. Both transient outward currents rose rapidly after a step depolarization, but their subsequent time course differed greatly. The fast-transient current decayed within 20 ms, while the slow-transient current took greater than 10 s to decay. Raised extracellular potassium reduced current amplitude. Both currents were present in cadmium-containing or calcium-free perfusate. 3. Tetraethylammonium had little effect on the slow-transient current at a concentration of 1 mM, but the fast-transient current was reduced by 60%. 4-Aminopyridine had little effect on the fast-transient current over the range 20 microM-2 mM, but these concentrations reduced the slow-transient current and altered its time course. 4. Both transient currents were evoked by depolarizations below action potential threshold. The fast-transient current was evoked by a 7 mV smaller depolarization than the slow-transient current, but its chord conductance increased less steeply with depolarization. 5. Voltage-dependent inactivation of the fast-transient was steeper than that of the slow-transient current (4 vs. 7 mV per e-fold change), and half-inactivation occurred at a less negative potential (-59 vs. -65 mV). The activation and inactivation characteristics of each current overlapped, however, implying the existence of a steady 'window current' extending over a range of approximately 14 mV beginning negative to action potential threshold. 6. The fast-transient current displayed a clear voltage dependence of both its activation and inactivation kinetics, whereas the slow-transient current did not. Recovery of either current from inactivation took about 1 s near -70 mV. The recovery of the slow-transient current became faster with hyperpolarization. 7. The contribution of each transient current to repolarization of the action potential was assessed from pharmacological responses. Blockade of calcium influx had little or no effect on the rate of action potential repolarization, whereas the selective reduction of either transient current caused significant slowing of repolarization. 8. We conclude that Betz cells possess at least two transient potassium currents, each a member of the rapidly expanding family of voltage-gated potassium currents that have been identified in various cell types.(ABSTRACT TRUNCATED AT 400 WORDS)