Patients with Obstructive Sleep Apnea Syndrome (OSAS) have elevated sympathetic drive and impaired parasympathetic cardiac control during wakefulness and sleep. Previous studies have demonstrated that sympathetic overactivity in OSAS can lead to impaired peripheral vasodilatory responses to hypoxia. The present study was conducted to determine whether the autonomic response to arousal from sleep is also altered in OSAS. In 8 OSAS and 8 normal controls, we performed time-varying spectral analyses of cardiovascular and respiratory variability following acoustically-induced arousal from sleep. In all subjects and in both REM and non-REM sleep, the arousal responses of the spectral parameters representing vagal heart-rate modulation were small or non-existent after accounting for the accompanying changes in respiration. In non-REM sleep, the OSAS subjects showed substantially attenuated cardiac sympathetic responses to arousal but the peripheral vascular response was preserved. In REM sleep, the sympathetic cardiovascular responses to arousal were similar in both subject groups, but these responses were much smaller than those exhibited by normals in non-REM sleep. Our results suggest that OSAS leads primarily to an impairment of the cardiac sympathoexcitatory reaction to arousal from non-REM sleep; however, the normal autonomic cardiovascular response to arousal in REM sleep appears to be preserved.