Studies post extubation in neonates recovening from respiratory disease have suggested that expiratory resistance (RE) plays a role in maintaining adequate lung volume. To evaluate effects of RE on PF, 6 previously intubated newborns who were breathing spontaneously in room air had PF measured after the application of 2 external expiratory resistances: EER1 = 30 cm H2O/L/sec and EER2 = 24 cm H2O/L/sec. PF tests including dynamic lung compliance (CL), inspiratory resistance (RI), RE, functional residual capacity (FRC), tidal volume (VT), and inspiratory:expiratory (I:E) ratio were studied in infants (mean wt. 2.26 kg., mean age studied 55 days [range 3-114] ) evaluated with a face mask, pneumotachograph, solenoid valve, and EER1 and EER2. Patients were studied at 4 phases: 0 resistance, EER1, 0 resistance, EER2. There was a mean 25.6% increase of FRC with EER1 (p<0.01) and mean 37.8% increase of FRC with EER2 (p<0.01). I:E ratio decreased by mean 16.0% (p<0.05) with EER1 and mean 22.6% (p<0.05) with EER2. CL, RI, and VT were unchanged at any phase. This study demonstrates that lung volume can be increased when an expiratory resistance is applied. Therefore, application of an expiratory resistance may prove to be useful in the prevention of atelectasis in neonates post extubation.