Hypopneas or pauses in respiratory effort frequently precede episodes of obstructive sleep apnea resulting in mixed apneas. We studied five subjects after chronic tracheostomy for obstructive sleep apnea. During stable non-REM (NREM) sleep, subjects breathed entirely through the tracheostomy. Tracheostomy occlusion caused experimental obstructive apnea which lasted 13.9 ± 4.7 sec and ended with transient arousal and pharyngeal opening. At the end of the apnea there was marked hyperventilation (inspired minute ventilation rose 21.6 ± 3.5 L on the first breath) followed by hypocapnia, hypopnea, and pauses in inspiratory effort as the subjects resumed NREM sleep. Hypocapnia was greater before inspiratory pauses lasting at least 5 sec than before shorter pauses (PETCO2, 4.2 ± 1.8 mm Hg below baseline vs 1.2 ± 2.5 mm Hg below baseline). In three patients, pauses in inspiratory effort following experimental obstructive apnea were prevented by administration of 4 percent CO2 and 40 percent O2 inspired gas. This study suggests that: 1) hyperventilation with hypocapnia occurs at the termination of obstructive apneas, and 2) hypocapnia may be responsible for the attenuation or cessation of respiratory effort initiating the subsequent cycle of obstruction.