The respiratory activity (EMGs) of the posterior cricoarytenoid (PCA), cricothyroid (CT) and diaphragm (D) were examined during slow wave sleep (SWS) and paradoxical sleep (PS).-Chronically implanted, free-to-move adult rats were used. In SWS, CT exhibited inspiratory or expiratory or inspiratory and expiratory bursts in each respiratory cycle. The latter was common during CO2 (4%) breathing. PCA manifested phasic inspiratory discharges along with tonic expiratory activity. The latter was augmented by breathing CO2. At onset of PS, inspiratory PCA and CT activity declined. In those PS epochs of irregular D activity, PCA and CT further declined during eye movements. In other PS epochs, D arrest coincided with co-activation of PCA and CT, occasionally CT activation alone. CO2 breathing did not affect the above described during PS. In PS, loss of PCA and CT's inspiratory activity during rhythmic D activity may contribute to obstructive apnea; PCA and CT co-activation with D arrest characterizes central type apnea.