This review concerns studies on the electrograms of respiratory muscles carried out in unrestrained sleeping cats. The respiratory unit discharges of the diaphragm and intercostal muscles undergo only quantitative changes from quiet wakefulness to synchronized sleep. Intercostal postural activity is also evident whereas such activity is practically absent in diaphragmatic electrograms. During desynchronized sleep the activity, both postural and respiratory, of intercostal muscles is tonically depressed and highly irregular, while diaphragmatic electrograms are scarcely affected, except for random disturbances related to the phasic events of this stage of sleep (rems, muscle twitches). The changes in the activity of intercostal muscles do not depend on modifications of the activity of respiratory centres as phrenic motor neurones are not tonically depressed. Only strong phasic influences of non-respiratory brain stem structures may affect phrenic motor neurones during desynchronized sleep. The depression of intercostal respiratory activity during this stage of sleep rather depends on the tonic inhibitory influences of brain stem structures on spinal motor neurones affecting also intercostal postural activity. The respiratory frequency during desynchronized sleep increases and decreases above eupneic and below polypneic values of synchronized sleep, respectively. Such a phenomenon cannot be related to the tonic brain stem inhibition of spinal motor neurones occurring during desynchronized sleep as it is unlikely that the same influence may elicit two opposite effects. These effects can be better explained on the basis of a release of respiratory centres from higher controls, particularly hypothalamic. In conclusion, the clear dichotomy in respiratory motor innervation between synchronized and desynchronized sleep reveals a basic change in respiratory regulation whose functional significance is still obscure.