A review of the effects of brain-stem transections, carbachol microinjections and lesions does not show that the pons is necessary and sufficient for paradoxical sleep (PS) generation. The different theories that explain the mechanisms of PS rebound are summarized. In some circumstances, PS rebound depends upon the stressful quality of PS deprivation. PS appears to necessitate oxidative metabolism of glucose and, in certain situations, PS may be increased by hyperoxia.