We studied the activity of the diaphragm and of the genioglossus at the onset and at the end of obstructive sleep apnea in children. Seven children (mean age 46 months, range 15–87) with obstructive sleep apneas mainly due to enlarged tonsils were tested during natural sleep. We recorded sleep stages (neurophysiological criteria), nasal and buccal air flow (thermistors), thoracoabdominal motion (magnetometers), genioglossus and diaphragm electromyographic activity (EMG) (surface electrodes), and transcutaneous partial pressure of oxygen (Radiometer 44° C). A total of 153 obstructive apneas for the whole group of patients was studied. Compared to the preceding unoccluded breaths, genioglossus and diaphragm EMG data showed that 1) at the onset of obstructive apnea there was no significant decrease in genioglossus and/or diaphragm EMG, contrasting with published data for obese adults, and 2) at the end of obstructive apnea, significant preferential increase in genioglossus EMG, not related to the decrease in transcutaneous partial pressure of oxygen, was found as in obese adults. This study showed that different mechanisms may control the onset of obstructive apnea in children as compared to adults, whereas children and obese adults share the same preferential increase in genioglossus EMG at the end of obstructive apnea.