1. Intracellular recordings were made from lumbar alpha-motoneurones in unanaesthetized decerebrate acute spinal cats. The response of motoneurones to direct current pulse injection or synaptic excitation was investigated following intravenous injection of L-beta-3,4-dihydroxyphenylalanine (L-DOPA, 20-120 mg/kg) alone, nialamide (10-50 mg/kg) and L-DOPA or clonidine (0.5-1 mg/kg). 2. The response properties of motoneurones were tested with rectangular and triangular current waveforms. Before L-DOPA treatment motoneuronal firing during a rectangular current pulse is characterized by an initial high firing frequency which rapidly decreases to a lower steady-state firing which is maintained only for the duration of the pulse. Following administration of L-DOPA an acceleration in firing frequency is apparent following the initial adaptation seen with rectangular current pulses. A transient after-depolarization or an after-discharge often followed the termination of the pulse. The frequency-current relation in response to a triangular current injection changed from a clockwise to a counter-clockwise hysteresis after L-DOPA treatment (i.e. after L-DOPA the firing frequency was higher for any given current during the descending phase than during the ascending phase of the triangular waveform). 3. Firing acceleration during and self-sustained firing after rectangular current pulses and counter-clockwise hysteresis of firing frequency with triangular current pulses are causally related to the presence of plateau potentials, which can be directly visualized after inactivation of the spikes. Plateau potentials in motoneurones could be generated by short-lasting intracellular depolarizing current pulses or brief excitatory synaptic inputs and terminated by short-lasting hyperpolarizing current pulses or brief inhibitory synaptic inputs. Plateau potentials were demonstrated in flexor and extensor motoneurones. 4. All bistable properties described in the preceding paragraphs following L-DOPA administration could also be seen after administration of the alpha-receptor agonist clonidine. 5. Slow rhythmic oscillations of the membrane potential (7.5-10 Hz) were seen superimposed on plateau potentials in a few cells after administration of L-DOPA and clonidine. The oscillations had an amplitude in the range 10-20 mV and represent the expression of an intrinsic property of the motoneurone. 6. It is demonstrated that plateau potentials in the motoneurones contribute to the late long-lasting reflexes observed in L-DOPA-treated spinal cats. 7. It is concluded that L-DOPA (and clonidine) change the response properties of the motoneurones in an analogous way to 5-hydroxy-DL-tryptophan (5-HTP).(ABSTRACT TRUNCATED AT 400 WORDS)