To determine the importance of arousals from sleep on alterations of the cardiovascular control associated with obstructive sleep apnea syndrome (OSAS) we measured heartrate, continuous blood pressure and respiration in 5 OSAS patients and 5 control subjects during test procedures performed in overnight sleep. The tests consisted of inducing arousal with acoustic stimuli randomly delivered when subjects were in a stable sleep stage. Time-varying systems identification by means of the Laguerre expansion technique allowed the tracking of arousal-induced changes in the impulse responses of the respiratory sinus arrhythmia (RSA) and baroreflex (ABR) components of the cardiovascular control model. In the control group, significant reductions in ABR gain were detected during arousal. These changes were not observed in the OSAS group. RSA gain increased in controls during arousal, but its time course in OSAS was found to be significantly different. The lack of ABR reaction to arousal in OSAS reflects an impaired baroreflex that results from chronic exposure to repeated apnea episodes usually accompanied by sleep disruption.