<正> Changes of the neuronal discharge of 128 medullary respiratory unitswere recorded and studied during the period of expiratory apnea induced reflexlyby intracarotid sinus injection of sodium citrate in rabbits.Generally

theneuronal discharge of inspiratory units began

stopped and recovered at the sametime with those of the phrenic nerve.But

about 5% the phase-spanninginspiratory units near the obex showed a different time course with the dischargeof the phrenic nerve.They fired continuously in a low frequency while thephrenic nerve was quiet.When increasing progressively and approaching to acertain level

the firing rate increased abruptly and at the same time phrenic nervebegan to fire.So it seemed that they acted as the pacemaker of inspiration.Comparison of the cycle-triggered histograms（CTH）of these inspiratory unitswith those of phrenic nerve showed clearly the above mentioned phasicrelationship.They started firing before the phrenic nerve

but they reached theirmaximal rate and then declined and stopped quite in accordance with the phrenicnerve.It is

therefore

reasonable to assume that the central mechanism of theswitch from expiratory apnea to inspiration may originate from this kind ofneurons.Most of the expiratory units show tonic discharges during the period of apneawith a higher discharge rate than normal and then the rate decreases just beforerecovery of phrenic firing.In addition

small portion of the expiratory units weredepressed as the phrenic discharge ceased.The function of these two differentkinds of neurons in the mechanism of development of respiratory rhythm is

apparently

different.