The authors tested the hypothesis that, in the paediatric population, the Hering-Breuer reflex would be provoked by the positive pressure inflation of mechanical ventilation (IPPV), and that the strength of the reflex would relate to the child's age and compliance of the respiratory system (CRS). During IPPV, changes in air flow, volume and airway pressure were recorded simultaneously. From the traces, expiratory times and CRS were calculated. The mean expiratory time (Te1) prior to each inflation and the mean expiratory time (Te2) immediately following an inflation were determined. The presence of the reflex was indicated by Te2 being longer than Te1, and the strength of the reflex was calculated as the difference between Te2 and Te1 expressed as a percentage of Te1. Twenty children were studied who had a mean age of 0.7 years (range 0.1-9.8 years). The reflex was provoked in 19 of the 20 patients and was not seen in the oldest patient. The strength of the reflex varied from 22 to 144% and was inversely related to postnatal age significantly (r = -0.73) but not CRS or inflation volume. It is concluded that the Hering-Breuer reflex is provoked during IPPV in young children; whether, as in neonates, this influences the efficacy of ventilation deserves investigation.