Introduction. Surfactant therapy in preterm infants with respiratory distress syndrome (RDS) is followed by a rapid improvement in oxygenation. In contrast, the concurrent increase in static pulmonary compliance (Crs) is less consistent and delayed, and pulmonary resistance(Rrs) seems unaffected. Since available information were obtained with conventional mechanical ventilation, we wanted to know whether Crs and Rrs were differently influenced by HFOV applying an early and individualized lung volume optimization (LVO) strategy. Additionally, the influence of antenatal steroids (AS) was examined. Methods. As HFOV was started, the continuous distending pressure (CDP) was increased stepwise until optimal CDP(OCDP) defined as that allowing good oxygenation with the lowest FiO2. We recently have shown that pre-surfactant LVO does not influence Crs and Rrs (J. Appl. Physiol, in press). Surfactant was indicated by an OCDP×FiO2 ≥ 3 and instilled in bolus without patient disconnection. Following instillation, FiO2 and CDP were adjusted so that the LVO strategy was applied thoroughly and Crs and Rrs were determined at post-surfactant hours (H) 1/2, 1, 2 and 6. Results. We analyzed data of 28 RDS infants (age 30.8±2 weeks) given surfactant. Mean pre-surfactant values were: OCDP×FiO2=5.4±1.5, Crs=0.47±1.3ml/cmH2O/kg and Rrs=0.16±0.03 ml/cmH2O/sec. Following surfactant, OCDP×FiO2 was rapidly decreased, reduction being significant at H1/2 (4.3±1.3) and thereafter. The first change in pulmonary mechanics was a sudden rise in Rrs(0.21±0.06 at H1/2) with a return back to pre-surfactant level beyond H2. Crs increase became significant from H2 (0.55±0.15), but still remained below normal level at H6 (0.64±1.4). Pre-surfactant OCDP×FiO2 tended to be lower in 9 infants exposed to AS, which was associated with a significantly higher Crs compared to non-AS patients. This difference was no longer observed after surfactant. Conclusion. Surfactant during primary HFOV allows a rapid decrease in ventilatory requirements. Comparatively, Crs improvement appears a protracted, even with a strategy promoting alveolar recruitment and minimizing lung injury. In patients meeting surfactant criteria, prenatal steroids do not significantly influence early post-surfactant evolution of respiratory parameters.