The peri-locus coeruleus alpha (peri-LCalpha) of the mediodorsal pontine tegmentum contains cholinergic and non-cholinergic neurons, and is critically implicated in the regulation of both wakefulness and paradoxical sleep (PS). The peri-LCalpha receives dense monoaminergic (adrenergic, noradrenergic, serotonergic, dopaminergic and histaminergic) afferent projections, but little is known about their exact roles in the control of sleep-wake cycles. We have therefore examined the in vivo effects of microdialysis application of monoamines to the peri-LCalpha and adjacent cholinergic and non-cholinergic tegmental structures on behavioural states and the electroencephalogram (EEG) in freely moving cats. Norepinephrine, epinephrine and dopamine selectively inhibited PS and induced PS without atonia when applied to the caudal part of the peri-LCalpha, which mainly contains non-cholinergic descending neurons, whereas histamine and serotonin had no effect at this site. In the rostral part of the peri-LCalpha and the adjacent X area (nucleus tegmenti pedunculopontinus, pars compacta), which contain many ascending cholinergic neurons, norepinephrine and epinephrine suppressed PS with a significant increase in waking and a decrease in slow-wave sleep, as expressed by a marked decrease in the power of the cortical and hippocampal delta (0.5-2.5 Hz) and cortical alpha (8-14 Hz) bands, and an increase in the cortical gamma (30-60 Hz) band. At these sites, histamine had similar waking and EEG-desynchronizing effects, but never suppressed PS, while dopamine and serotonin had no effect. These findings indicate a special importance of the adrenergic, noradrenergic and dopaminergic systems in the inhibitory or permissive mechanisms of PS, and of the adrenergic, noradrenergic and histaminergic systems in the control of behavioural and EEG arousal.