Both end-inspiratory (EIO) and end-expiratory (EEO) airway occlusions are used to calculate the strength of the Hering-Breuer inflation reflex (HBIR) in infants. However, the influence of the timing of such occlusions is unknown, as is the extent to which changes in volume within and above the tidal range affect this reflex. The purpose of this study was to compare both techniques and to evaluate the volume dependency of the HBIR in healthy, sleeping infants up to 1 yr of age. The strength of the HBIR was expressed as the ratio of expiratory or inspiratory time during EIO or EEO, respectively, to that recorded during spontaneous breathing, i.e., as the "inhibitory ratio" (IR). Paired measurements of the EIO and EEO in 26 naturally sleeping newborn and 15 lightly sedated infants at approximately 1 yr showed no statistically significant differences in the IR according to technique: mean (95% CI) of the difference (EIO - EEO) being -0.02 (-0.17, 0.13) during the first week of life and 0.04 (-0.14, 0.22) at 1 yr. During tidal breathing, a volume threshold of approximately 4 ml/kg was required to evoke the HBIR. Marked volume and age dependency were observed. In newborn infants, occlusions at approximately 10 ml/kg during sighs always resulted in an IR > 4, whereas a similar response was only evoked at 25 ml/kg in older infants. Age-related changes in the volume threshold may reflect maturational changes in the control of breathing and respiratory mechanics throughout the first year of life.