Positive end expiratory pressure (PEEP) is routinely used when ventilating preterm infants, and high levels are recommended in those with severe respiratory distress syndrome (RDS). Elevation of PEEP increases lung volume, as does surfactant administration. We postulated that in surfactant-treated infants even modest PEEP levels could result in overdistension and (CO(2)) retention. To test that hypothesis, lung volume, compliance and arterial blood gases were measured in eight preterm infants (median gestational age 28 weeks, range 26-35 weeks) at three PEEP levels. The infants, all with RDS, were studied at a median time of 18 h, (range 12-68 h) after their last dose of surfactant. Infants were routinely nursed at 3 cmH(2)O of PEEP, the PEEP level was then raised to 6 cmH(2)O or lowered to 0 cmH(2)O in random order. The new setting was maintained for 20 min; the PEEP level was then changed to the third level (0 or 6 cmH(2)O) again for 20 min. At the end of each 20-min period, lung volume, compliance and blood gases were measured. Lung volume was assessed by measuring functional residual capacity (FRC) using a helium dilution technique. Compliance was measured by relating the volume change from a positive pressure inflation maintained until no further volume change occurred to the pressure drop (peak inflating pressure PEEP). Increasing PEEP from 0 to 3 cmH(2)O and particularly to 6 cmH(2)O resulted in increases in FRC (P < 0.05), oxygenation (ns) and paCO(2) (P < 0.02). Specific compliance (compliance/FRC) (P < 0.05) and pH (P < 0.02) fell. CONCLUSION: Following surfactant treatment, relatively low levels of positive end expiratory pressure (</=3 cmH(2)O) may be appropriate.