Unit activity in the magnocellular basal forebrain nucleus was examined to characterize discharge patterns during synchronized and desynchronized electroencephalogram. Two types of basal forebrain neurons were identified by their firing pattern under urethane anaesthesia: bursting and tonic neurons. Bursting neurons (62.9%) were characterized by a spontaneous firing that consisted of periodic bursts of two to six spikes that occurred at 0.3 to 2 Hz and were phase-locked with the electroencephalogram slow waves. Tonic neurons (37.1%) displayed spontaneous single spike firing at 12.1 + or - 1.6Hz. The firing of most of them was not related to the slow waves. Both neuronal types changed their firing patterns during the electroencephalogram desynchronization elicited by either electrical stimulation of the pedunculopontine tegmentum or pinching the rat's tail. Bursting neurons changed from the bursting mode to a tonic mode of discharge pattern, increasing their firing rate, while tonic cells were inhibited during electroencephalogram desynchronization. Multiunit recordings revealed that bursting cells discharged synchronously during periods of electroencephalogram slow waves, but that synchronization disappeared during electroencephalogram desynchronization. No correlation was found between the spike discharges of different tonic cells nor between bursting and tonic cells. However, bursting neurons, but not tonic neurons, were correlated with the spike firings of neocortical neurons during electroencephalogram slow waves. The rhythmic activity of neither neocortical nor bursting basal forebrain cells was found under pentobarbital anaesthesia. The characteristics of the discharge pattern shown by bursting basal forebrain neurons suggest that this type of cell could be cholinergic. Thus, bursting basal forebrain neurons may release acetylcholine in the cortex rhythmically, enhancing the rhythmic activity of cortical neurons during slow-wave sleep. It is concluded that basal forebrain neurons may contribute to the generation of the electroencephalogram slow waves.