This model study compared the imposed work of two nasal continuous positive airway pressure (NCPAP) devices for very low birthweight (VLBW) babies: a new NCPAP device, designed by Moa et al., (Crit Care Med 16:1238, 1988), and a conventional NCPAP device. In addition, the variabilities in the pressures of a simulated airway were compared. A continuous flow (8 L/min) was used to generate CPAP. A Harvard ventilator was used to simulate breathing at a fixed volume (12.1 mL) and rate (45 breaths/min). The pressure drop across and the flow rate through each device were measured, thus allowing the determination of imposed work, i.e., the work of breathing done by the patient to overcome frictional losses due to the NCPAP device. In addition, the pressure fluctuations in the simulated airway were measured. The data presented are averages of 36 breaths using the new device and 34 breaths using the conventional device. The means of the imposed work in the new device and in the conventional device were 0.135 (95% CI +/- 0.004) mJ/breath and 0.510 (95% CI +/- 0.004) mJ/breath (P < 0.01) respectively. The coefficients of variation for pressure in the simulated airway were: new 6.8% and conventional 15.3%. We conclude that the imposed work of the new NCPAP device for the VLBW baby is approximately one-fourth of that of the conventional device. The airway pressure generated by the VLBW size of the new NCPAP device shows less variability during simulated breathing than that found with the conventional device. The findings support the clinical use of the new NCPAP device in the very low birthweight baby.