Respiration, R-R interval (RRI), blood pressure (BP) and other polysomnographic variables were recorded in 8 normals and 9 patients with untreated obstructive sleep apnea syndrome (OSAS) in wakefulness, stage 2 and rapid eye-movement sleep. A preprogrammed ventilator produced randomized breathing sequences, which induced corresponding RRI and BP fluctuations, without the need for voluntary control. Using a Volterra-Wiener model and the Laguerre expansion technique, we estimated the parameters that characterize linear as well as the second order effects of respiration ("RSA") and arterial blood pressure ("ABR") on heart rate. RSA and ABR gains were significantly lower in OSA than normals. During sleep, ABR gain increased in normals but remained unchanged in OSA. Summary statistics and spectral analysis also indicated reduced parasympathetic and elevated sympathetic activity in OSAS. The nonlinear effects of respiration and especially blood pressure on heart rate fluctuations were also suppressed in OSAS patients.