1. Electrical characteristics of the cell body of an identified motoneurone, the 'fast' coxal depressor motoneurone (Df), from the cockroach (Periplaneta americana) have been studied under current- and voltage-clamp. 2. In response to low magnitude, relatively long duration depolarising current pulses, Df could generate plateau potentials, regenerative events which often far outlived the duration of the applied depolarisation. 3. Plateau potentials constitute an inherent property of the neurone because they could be evoked in somata that had been surgically isolated from other parts of the neurone (the soma is devoid of synaptic contacts); these experiments also demonstrated that the soma of this neurone can participate in the generation of plateau potentials. 4. Plateau potentials were often surmounted by attenuated action potentials; these correlated 1:1 with axonal impulses recorded extracellularly from the axon of the neuron. 5. Plateau potentials were associated with an increase in membrane conductance. Under voltage- clamp, cells which exhibited plateau potentials possessed a region of negative slope resistance in their current-voltage relationship. 6. Plateau potentials in Df were observed to be calcium-dependent, A series of current- and voltage- damp experiments indicated that the calcium channels involved in plateau potential production differ from those which can mediate calcium-dependent action potentials following pharmacological treatment of this neurone. 7. Plateau potential production in Df was suppressed by the application of GABA (10-4M). Spontaneous plateau potentials could be recorded following application of picrotoxin (10-5M) or pentylenetetrazole (25mM). 8. Recordings taken from two other 'fast' motoneurones, cell 3 (from the cockroach) and FETi (from the locust, Schistocerca gregaria) indicated that the ability to generate plateau potentials may not be restricted to Df. 9. Although freshly dissected, recently impaled neurones responded to relatively brief depolarising current pulses with a series of graded, damped membrane oscillations, the excitability of many preparations increased with time from dissection: many cells became able to generate all-or-none action potentials in response to such pulses (these differed from the attenuated axonal spikes which often surmounted plateau potentials). The appearance of these events did not correlate with consistent changes to the resting potential or input resistance of neurones. 10. Time-dependent action potentials were calcium-dependent and could be recorded from 'intact' cells and isolated neurone somata. These action potentials could also co-exist with plateau potentials; such co-existence provides evidence for different classes of calcium channel in untreated insect neurones.