To minimize respiratory effort, continuous positive airway pressure (CPAP) must be applied in a manner that maintains a constant airway pressure. Conventional CPAP circuits depend on relatively high fresh gas flows (FGF) to maintain circuit pressure during inspiration. The authors describe a CPAP circuit employing a "weighted bellows." This simple modification of the conventional gas reservoir allows the FGF to be significantly reduced. In fact, minimal changes in circuit pressure can be achieved with FGF that barely exceeds the patient's minute ventilation. The modified system is compact, mechanically uncomplicated, and simply constructed.