The effects of microinjection of glycine and strychnine into unilateral and bilateral Bötzinger complex (Böt. C) on phrenic nerve discharges were observed in 34 urethane anaesthetized, vagotomized, paralyzed and artificially ventilated rabbits. The results are as follows: (1) microinjection of glycine into unilateral Böt. C induced an increase of respiratory rate and expiratory phrenic nerve discharges; (2) microinjection of glycine into bilateral Böt. C induced non-rhythmic tonic phrenic nerve discharges with an amplitude 40%-70% of control; strychnine could competitively antagonize the effects of glycine; and (3) microinjection of strychnine into bilateral Böt. C induced a decrease of respiratory rate and the amplitude of phrenic nerve discharges, in addition to prolongation of expiratory duration while inspiratory duration remained unchanged. The above results suggest that the Böt. C plays a crucial role in initiating and sustaining expiration.