The effect on blood gases of different ventilator rates in preterm infants ventilated beyond the first week of life was assessed. Seventeen infants, median gestational age 25 weeks, were studied at median postnatal age of 11 days. The infants were ventilated through a set sequence of rates: 30, 60, 30, 100 and 30 breaths per min (bpm), each rate being maintained for 20 min. Peak and positive end expiratory pressure and I:E ratio (1:1) were unchanged at each rate and mean airway pressure was kept constant by altering flow as necessary. No significant change in oxygenation was demonstrated at either rates of 60 or 100 bpm compared to 30 bpm. PaCO2 levels were, however, significantly reduced at 60 bpm (P less than 0.001) compared to 30 bpm; but this improvement in PaCO2 was not seen at 100 bpm. These results suggest that increasing ventilator rate higher than 60 bpm in the majority of infants ventilated after the first week of life is not advantageous.