Large-amplitude waves recorded in the pontine tegmentum, lateral geniculate body and visual cortex herald the onset and continue throughout paradoxical sleep. The role of these ponto-geniculo-occipital (PGO) waves, or spikes, has puzzled researchers since their discovery. This paper reports experiments in cats which have demonstrated that PGO spikes are essentially an epiphenomenon, an electrical sign of the activation of a "startle network" by the neural turmoil of paradoxical sleep. Internal stimulation provided by the bursts of neural activity which characterize paradoxical sleep produces PGO spikes in the lateral geniculate body which are identical in appearance to those elicited during synchronized and paradoxical sleep by 1.55 Hz tone bursts or taps on the cage in normal cats. Cerebellar lesions result in behavioral responses to the intrinsic startles during synchronized sleep in the form of extensor or flexor jerks of the forelimbs. The jerks occur in conjunction with each PGO spike. Identical movements can be induced in the same cats in wakefulness by such startling stimuli as dropping the cat or hissing with an aerosol can. Lesions involving the auditory-visual area permit cats to be stimulated by sound in synchronized sleep without arousal, but anterior lobe lesions produce an easily aroused animal. We postulate that the phenomena observed following cerebellar lesions are the result of alteration in the control of serotonergic neurons of the pontine raphe nuclei.