We have measured the ventilatory response to acoustically induced arousal in normal subjects and patients with obstructive sleep apnea syndrome (OSAS). The arousal responses are similar in magnitude and time-course over the first 3 breaths, but in OSAS the subsequent response declines much more rapidly. Incorporation of these empirical findings into an existing model of sleep-disordered breathing allows an improved characterization of state-chemoreflex interactions. The shorter time-course of the arousal response in OSAS promotes greater ventilatory and state instability at low-to-intermediate levels of CO2 gain.