The influence of postnatal age on spontaneous respiratory time, measured both on and off continuous positive airways pressure (CPAP), and the occurrence of the Hering Breuer reflex has been investigated. Consecutive ventilated infants were recruited and only studied when making respiratory efforts during mechanical ventilation. Fifty-three infants were studied on 119 occasions, their median gestational age was 29 weeks and birthweight 1142 gms. All the infants were initially ventilated for the respiratory distress syndrome. Inspiratory and expiratory times were measured from simultaneous flow and oesophageal pressure recordings during a 20-second period of disconnection and during a similar period on 3 cmH2O CPAP. Prolongation of the inspiratory and expiratory time on CPAP compared to that measured during disconnection of more than 10% was evidence of the Hering Breuer reflex. No significant change in the inspiratory or expiratory time either on or off CPAP was noted either between days 1 to 5 or weeks 1 to 4. Both inspiratory and expiratory time tended to be longer on CPAP than during disconnection and these differences reached significance on days 1 and 2 and weeks 1 and 2. No influence of postnatal age was demonstrated on the proportion of infants in whom the Hering Breuer reflex was demonstrated. Our results suggest that, during the neonatal period, there is no influence of postnatal age on respiratory timings or reflex activity amongst ventilated infants who make spontaneous respiratory efforts during ventilation. These results have important implications regarding the optimum rate at which such infants should be ventilated.