In 8 pigs and 4 baboons, spontaneously breathing and anesthetized with halothane, Hering-Breuer reflex was tested by total obstruction of the airway, preventing either inspiration or expiration. Subsequently animals were paralyzed and maintained on a phrenic nerve driven servo-respirator. The response of phrenic motoneuron output to various degrees of lung inflation, introduced for one breath only was carefully studied. This was achieved by varying the gain of the servo-respirator. In baboons an identical series of gain maneuvres was performed against a background of different levels of the initial gain setting. Changes in inspiratory time and peak amplitude of phrenic signal were monoexponentially dependent on the gain of the servo-respirator and linearly dependent on tidal volume (all negatively correlated). The relationship between inspiratory time, TI, and subsequent expiratory duration, TE, existed only within a range of growing TI. Vagal positive feedback phenomenon was apparent in pigs and negligible in baboons. Apparently the inspiratory cut-off mechanism terminates inspiration when excitatory function are outbalanced by their integral.