Because the behavior of right ventricular stroke volume (RVSV) in the obstructive sleep apnea syndrome (OSAS) is undefined, we studied the time course of RVSV by right heart catheterization during sleep in five OSAS patients. In 55 obstructive apneas, heart rate (HR) and RVSV were calculated beat-by-beat. RVSV was estimated by integrating the area under the pulmonary arterial (PA) blood velocity signal obtained by a velocity sensor-/micromanometer-tipped catheter. Compared with preapnea, mean RVSV did not change significantly during apnea, but decreased by 15% during postapnea. Analysis of RVSV in the respiratory cycle showed that postapneic RVSV was mostly reduced at maximal inspiration, suggesting a role of increased lung volume in decreasing RVSV. As for HR, it decreased significantly during apnea in four out of five patients. In all patients, HR returned to preapneic values during postapnea. Therefore, right ventricular output decreased slightly in most patients during late apnea because of decreased HR, whereas it decreased in all patients during postapnea because of decreased RVSV. These results, together with the known finding that PA pressure increases towards the end of apneas and remains elevated in the immediate postapnea, suggest that the most relevant changes in the pulmonary circulation occur at the resumption of ventilation, not during the apneic phase.