Bilateral lesions of the pontine gigantocellular tegmental field in the cat resulted in the complete elimination of paradoxical sleep during 3 weeks postoperative recording. The tonic muscular atonia, normally characteristic of this state, was absent. The phasic components, rapid eye movements and ponto-geniculooccipital (PGO) spikes, did not occur in association with an activated EEG, as they normally do in paradoxical sleep. In fact, PGO spikes were virtually absent immediately after the lesion and were only secondarily apparent as isolated phenomena during slow wave sleep to represent in total daily number 5% of normal the first week and 15% of normal the third week after the lesion. These results indicate that neurons whose perikarya and/or processes are located within the pontine gigantocellular tegmental field and which are not part of the noradrenaline locus coeruleus complex, are critical for paradoxical sleep.