1. In the preceding paper (Crone, Hultborn, Kiehn, Mazieres & Wigström, 1988) it was shown that a short-lasting synaptic excitation ('on' stimulus) of extensor motoneurones (primarily triceps surae) in the decerebrate cat often resulted in a maintained excitability increase, which could be reset by a short-lasting inhibitory stimulus train ('off' stimulus). In the present experiments intracellular recording from triceps surae motoneurones and the electroneurogram (ENG activity) from triceps surae nerve branches were performed in parallel. 2. Sustained firing of individual triceps surae motoneurones was most often recorded in parallel with the maintained ENG activity following a synaptic 'on' stimulus. When the motoneurone was silenced, by a hyperpolarizing current through the microelectrode, there was no sign of on-going synaptic excitation during the maintained ENG activity following an 'on' stimulus. It was therefore suggested that voltage-dependent intrinsic properties of the motoneurones themselves could be responsible for the maintained firing. 3. In confirmation of this hypothesis it was found that short-lasting depolarizing current pulses through the recording microelectrode could trigger a self-sustained firing in the motoneurone provided that the bias current (i.e. the holding potential) was kept within certain limits. Hyperpolarizing current pulses terminated the firing. When the spike-generating mechanism was inactivated (by long-lasting excessive depolarization) similar depolarizing and hyperpolarizing current pulses could initiate and terminate plateau potentials in the motoneurones. By grading the depolarizing current pulses it was found that the plateau potentials were of all-or-none character, typically around 10 mV in amplitude. The two levels of excitability which can be triggered by short-lasting excitation and inhibition of the motoneurones is referred to as 'bistable' behaviour of the motoneurones. 4. After an acute spinal transection, in the unanaesthetized cat, the bistable behaviour of the motoneurones disappeared. However, it reappears following intravenous injection of the serotonin precursor 5-hydroxytryptophan (50-120 mg/kg). 5. Individual triceps surae motor units were recorded by selective EMG electrodes during tonic stretch reflexes in the decerebrate preparations. Based on an analysis of their firing pattern during lengthening and shortening (or vibration) of the muscle it is suggested that plateau potentials in motoneurones are recruited during the tonic stretch reflex. Furthermore, it is argued that a quantitatively important part of the depolarization of motoneurones during the tonic stretch reflex indeed originates from these plateau potentials.(ABSTRACT TRUNCATED AT 400 WORDS)