During continuous positive airway pressure (CPAP) the work of breathing is least when there is no change of airway pressure measured at the mouth (delta Paw) during respiration. In this study we consider the physical properties of CPAP circuits which determine the magnitude of delta Paw, and apply this information to the design of a simple, reliable CPAP system that minimizes delta Paw and incorporates the additional advantages of low-flow rates and the use of commonly available proven components. Using a weighted reservoir bag and threshold CPAP valve to maintain pressure, we found that delta Paw may be reduced to less than 1.0 cm H2O during quiet breathing, at a tidal volume of 700 ml and maximum inspiratory flow of 1.35 L/sec. delta Paw was dependent on both the resistance and the reactance of the circuit components; it was increased when resistance or flow rates were high, and when delivery pressure depended on the elastic tension in the wall of the reservoir bag or on compression of the bag with springs or elastic bands.