To document mechanisms contributing to upper airway collapse, we compared the electromyographic activity of the genioglossus (GG) and diaphragm (DIA) during spontaneous mixed and obstructive apnea and during induced end-expiratory airway occlusion in 11 premature infants. In addition to ventilation and esophageal pressure measurements, we obtained DIA and GG electro-myograms (EMG) from subcostal and sublingual surface electrodes, respectively. Amplitude of the DIA EMG and the frequency of occurrence of the GG EMG were determined for: I) the breath preceding apnea or occlusion, 2) the initial and last obstructed inspiratory efforts, and 3) the first breath at resolution of both apnea and occlusion. During spontaneous apnea with airway obstruction amplitude of the DIA, EMG decreased on the initial obstructed inspiratory effort and did not exceed that of the breath preceding apnea until reestablishment of flow. In contrast, during end-expiratory airway occlusion, the amplitude of the DIA EMG increased both during and at release of occlusion. In 18 ± 6% of the spontaneous apneic episodes, GG EMG was present with the breath preceding apnea and this frequency did not increase significantly until resolution of the apnea. During induced airway occlusion, GG EMG was not present with the breath preceding occlusion but its frequency did increase to 58 ± 8 and 42 ± 8% with the last occluded inspiratory effort and the first breath after release of occlusion, respectively. The decreased presence of the GG EMG from the last occluded effort to the breath at release of occlusion (58 ± 8 versus 42 ± 8%, p < 0.05) was probably due to the greater mechanoreceptor-mediated inhibition associated with reestablishment of flow. This study thus demonstrates markedly different respiratory muscle responses to spontaneously occurring obstructed inspiratory efforts during apnea as compared to experimentally induced airway occlusion in preterm infants. © 1989 International Pediatric Research Foundation, Inc.