Our studies suggest that paradoxical sleep is a state in which the brainstem is in a functional mode normally associated with presentations of novel stimuli. Furthermore, the combination of atonia and an internal state of activation indicates sustained activity of a brainstem mechanism designed to dampen responses to sudden, novel stimuli in wakefulness lest the animal over react and run blindly into danger prior to stimulus analysis. This conclusion stems from two sets of data. Studies of large-amplitude waves, which characteristically occur spontaneously just prior to and during paradoxical sleep, have demonstrated that the waves are signs of alerting. These waves, termed ponto-geniculooccipital (PGO) spikes, can be induced during both slow wave and paradoxical sleep by external stimuli at a threshold below actual arousal. Whenever cats are confronted with novel stimuli during wakefulness, eye movement potentials, which are recorded from the same sites as PGO spikes but differ from them in several characteristics, assume all the characteristics of PGO spikes. These observations indicate that the central nervous system during paradoxical sleep is in a "peculiar" state of activation which is not behaviorally expressed. Other experiments have focused on the muscle atonia of paradoxical sleep. Small, bilateral dorsolateral pontine tegmental lesions create the dramatic phenomenon of paradoxical sleep without atonia which is characterized as follows: After slow wave sleep, when paradoxical sleep with muscle atonia would normally appear, cats raise their heads, make body righting movements, exhibit alternating movements of the limbs, and even attempt to stand. Throughout an episode, which shows all other aspects of paradoxical sleep, including unresponsiveness to visual stimuli, cats act as if they are being startled, searching and sometimes attacking an object. In wakefulness they show minor cerebellar signs. Presumably the lesions disrupt both the pontine excitation of the medullary inhibitory area and the inhibition of a brainstem system for mobilizing activity normally in force during paradoxical sleep. Studies of the same cats during wakefulness have shown that there in an increase in exploratory locomotor activity of 23 to 127 percent as measured in an open-field test. The existence of parallel effects on motor control in wakefulness and paradoxical sleep produced by pontine lesions suggests that the atonia of paradoxical sleep is a reflection of excessive activity of a brainstem response dampening mechanism which operates more subtly during wakefulness to produce appropriately modulated responses to various unexpected stimuli.