Inspiratory and expiratory volumes were measured in 51 preterm infants with respiratory distress syndrome (RDS), when comparing two ventilator rates, 60 and 120 breaths/min. Gas trapping was not demonstrated at rates of 60, but in 11 infants at 120 breaths/min and this was more common in the paralysed infants and those more mature than 31 weeks gestational age ( P < 0.05). The median change in functional residual capacity resulting from gas trapping was 3.8 ml/kg. We conclude rates of 120 breaths/min can be used in the majority of non-paralysed infants without gas trapping but should be avoided in paralysed infants more mature than 31 weeks.