Initial observations on healthy term infants using the forced partial expiratory flow-volume technique with an inflatable jacket suggested that this technique was stimulating the Hering-Breuer deflationary reflex, a reflex which has not been systematically studied in man. To investigate this fully, esophageal pressure, jacket pressure, flow and volume at the mouth were recorded during the forced partial expiratory flow-volume maneuver on 10 infants (mean age, 3.1 days; birth weight, 3533 g; gestation, 39.8 weeks). A total of 186 measurements were performed at different points in the respiratory cycle. In 159 (85%) recordings inspiratory effort was evident with a fall in esophageal pressure within 166 msec; in some cases this occurred before the jacket was fully inflated. This was associated with a reduction of 23.4 cm H2O in mean intrathoracic pressure, which was 2.5 times that occurring during normal tidal breathing. In the remaining 27 measurements a plateau pressure was associated with closure of the upper airway. When the squeeze was applied at low lung volumes (end-expiration) the inspiratory effort occurred significantly earlier (133 msec) and stronger (reducing peak intrathoracic pressure to 15.8 cm H2O) than when applied at end-inspiration (181 msec with a reduction in intrathoracic pressure to 25.2 cm H2O). The observed inspiratory response was highly consistent, representing the deflationary reflex as described by Hering and Breuer in 1868. The stronger and more rapid onset of inspiration at low lung volume supports the claim made by Breuer that it has a protective role on functional residual capacity (FRC) in young infants.