Despite early suppositions that the noradrenergic (NA) locus coeruleus (LC) neurons play a critical role in the generation and tonic maintenance of wakefulness and paradoxical sleep, further studies indicated that these cells play a nonessential modulatory role in the regulation of these states. Thus, based upon evidence from pharmacological, lesion and single-unit recording studies, it now appears that NA neurons may be important for enhanced periods of attention or stress during wakefulness, though they are not necessary for the tonic maintenance of cortical activation or behavioral arousal during the state. From similar examinations, it has been found that the cessation of activity of NA LC neurons may normally be important in permitting the occurrence of the state of paradoxical sleep. Neighboring cholinergic neurons of the pontomesencephalic tegmentum may also be active during waking and play a role in facilitating thalamocortical activity and transmission, like NA neurons during that state. However, unlike the NA neurons, the cholinergic neurons play an active and essential role in the generation of the state of paradoxical sleep. Generation of the state of paradoxical sleep may depend upon the simultaneous activation of cholinergic neurons and cessation of NA LC neurons, that could be brought about by the intermediary action of local GABA neurons.