Continuous positive airway pressure (CPAP) breathing can be delivered using the demand-flow (DF) or continuous-flow (CF) system. A modified CF system, the flow-by (FB) system, operates with preset base-flow (BF) values between 5 and 20 L/min. The DF depends on changes in pressure for opening the pneumatic valve of the system (pressure sensitivity). In contrast, the FB depends on changes in flow (flow sensitivity). In six healthy male subjects, we determined the mechanical inspiratory work of breathing (WI, J/L) and the inspiratory work rate (J/min) on the DF and the FB systems at a BF of 5 and 20 L/min, at CPAP levels of 0, 5, and 10 cm H2O. In comparison to DF, on the FB system both WI and inspiratory work rate were significantly less at a CPAP of 10 cm H2O (p less than .01). This was most likely due to the smaller drop in airway pressure at the onset of inspiration with the FB system. Varying the BF values in the FB system had no effect on WI or inspiratory work rate.