Obstructive apneas occur infrequently during Stage 3-4 NREM sleep (SWS), even in patients with severe obstructive sleep apnea. To investigate whether upper airway (UA) dilator muscle activity preferentially increases during SWS as a partial explanation for this phenomenon, we measured phasic electromyogram activity of the genioglossus muscle (EMGgg) during continuous Stage 2 NREM sleep and SWS in 5 healthy males. Subjects were studied supine during a complete cycle of nocturnal NREM sleep after partial sleep deprivation. EMGgg was measured with perorally inserted bipolar electrodes, and quantified as peak phasic inspiratory activity during all continuous epochs of NREM sleep. We found EMGgg to be increased during SWS relative to stage 2 sleep by a mean of 58% among all subjects (P = 0.02); neither end-tidal PCO2 nor inspired minute ventilation varied between these sleep stages. Upper airway resistance, measured in 3 of the subjects on a separate study night, was not different between SWS and Stage 2 sleep. We speculate that the increase in phasic EMGgg during SWS in our normal subjects may reflect a mechanism whereby UA patency tends to be preserved during this stage.