1. The properties of voltage-activated K+ currents were examined using whole-cell recording techniques in acutely isolated chick ciliary ganglion neurones. 2. Application of depolarizing voltage pulses from a holding potential of -60 mV evoked sustained outward currents that inactivated with time constants of hundreds of milliseconds (IDR). Bath application of 10 mM tetraethylammonium (TEA) caused a 70-90% reduction of IDR. Application of depolarizing voltage steps from a holding potential of -120 mV revealed a second class of TEA-resistant outward currents. These currents activated quickly but inactivated completely within tens of milliseconds (IA). IA activated at more negative command potentials than IDR. However, IDR exhibited a steeper voltage dependence of activation than IA. 3. The midpoint of the steady-state inactivation curve of IA was between -95 and -110 mV. By contrast the midpoint of the steady-state inactivation curve of IDR was between -80 and -90 mV. It was not possible to produce a complete inactivation of IDR using prepulses of up to 2 s duration. 4. The time course of IA inactivation could only be fitted with double-exponential curves with time constants of 5-18 ms and 30-60 ms at membrane potentials positive to -30 mV. The inactivation of IA was slower at more positive membrane potentials because of a greater contribution of the long time constant. The individual time constants were not markedly voltage dependent. 5. Bath application of 5 mM 4-aminopyridine (4-AP) caused a 70-100% block of IA whereas 1 mM 4-AP was ineffective. Bath application of 560 nM alpha-dendrotoxin (DTX) produced a 50-70% reduction of IA, but application of 280 nM DTX had no effect on IA. 6. Application of 1 mM 4-AP produced a reversible 55-80% block of IDR measured at the end of a 500 ms depolarizing pulse. The 4-AP-sensitive components of IDR activated rapidly and exhibited a gradual inactivation with continued depolarization. The 4-AP-resistant components of IDR activated much more slowly and showed very little tendency to inactivate. Significant blockade of IDR was produced by 10 microM 4-AP. 7. The decay of IDR tail currents could only be fitted with double exponential curves with time constants of 3-6 and 40-60 ms, respectively. The fast and slow components of the tail currents behaved independently with respect to the duration of the depolarizing voltage step. 8. Application of 1 mM 4-AP eliminated the fast, but not the slow component of IDR tail currents.(ABSTRACT TRUNCATED AT 400 WORDS)