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Respiration, R-R interval, blood pressure, and other polysomnographic variables were recorded in eight normal subjects and nine patients with untreated obstructive sleep apnea syndrome in wakefulness and sleep. To increase respiratory and cardiovascular variability, a computer-controlled ventilator delivered randomly modulated inspiratory pressures that were superimposed on a baseline continuous positive airway pressure. A mathematical model allowed heart rate variability to be partitioned into a component mediated by respiratory–cardiac coupling and one mediated by the baroreflexes. Respiratory–cardiac coupling gain was lower in patients versus normal subjects (36.9 ± 3.3 versus 66.1 ± 5.6 milliseconds L⁻¹, p < 0.03). Baroreflex gain in patients was also depressed relative to normal subjects (2.3 ± 0.4 versus 4.9 ± 0.7 milliseconds mm Hg⁻¹; p < 0.02). Baroreflex gain increased two- to threefold from …