To determine the long-term effects of continuous positive airway pressure (CPAP) therapy on cardiovascular variability, we measured R-R interval (RR), systolic blood pressure (SBP) and respiration (DeltaV) in 13 awake, supine patients with moderate-to-severe obstructive sleep apnea (OSA), before and after ~6 mo of treatment. Using these data, we estimated the dynamics of the following components of a closed-loop circulatory control model: 1) the baroreflex component, 2) the neural coupling of DeltaV to RR or respiratory sinus arrhythmia (RSA), 3) the mechanical effects of respiration (MER) on SBP, and 4) the circulatory dynamics (CID) component, which is responsible for the feedforward effect of RR fluctuations on SBP. Baroreflex and RSA gains increased whereas MER and CID gains decreased in compliant subjects whose average CPAP use was >3 h/night. In contrast, baroreflex, RSA, and MER gains remained unchanged and CID gain increased in noncompliant subjects. Other summary measures were unchanged in both groups, except for mean RR, which increased in compliant patients. Closed-loop analysis provides a simple but sensitive means for quantitatively assessing cardiovascular control in OSA by using data collected from a single, nonintrusive test procedure.