The response to chemical stimuli (chemical responsiveness), and the increases in respiratory drive required for arousal (arousal threshold) and for opening the airway without arousal (effective recruitment threshold) are important determinants of ventilatory instability and, hence, severity of obstructive apnea. We measured these variables in 21 obstructive apnea patients (apnea-hypopnea-index 91±24hr^-1) while on continuous-positive-airway-pressure. During sleep, pressure was intermittently reduced (dial-down) to induce severe hypopneas. Dial-downs were done on room air and following 30seconds of breathing hypercapneic and/or hypoxic mixtures, which induced a range of ventilatory stimulation prior to dial-down. Ventilation just before dial-down and flow during dial-down were measured. Chemical responsiveness, estimated as the % increase in ventilation during the 5^th breath following administration of 6%CO₂ combined with 4% desaturation, was large (187±117%). Arousal threshold, estimated as the % increase in ventilation associated with a 50% probability of arousal, ranged from 40% to >268% and was <120% in 12/21 patients, indicating that in many patients arousal occurs with modest changes in chemical drive. Effective recruitment threshold, estimated as % increase in pre-dial-down ventilation associated with a significant increase in dial-down flow, ranged from zero to >174% and was <110% in 12/21 patients, indicating that in many patients reflex dilatation occurs with modest increases in drive. The two thresholds were not correlated. Conclusions: In most OSA patients, airway patency may be maintained with only modest increases in chemical drive but instability results because of a low arousal threshold and a brisk increase in drive following brief reduction in alveolar ventilation.