Objective To determine whether 5-HT1A receptors could modulate the discharge activities of biphasic expiratory neurons(BE-neurons)and inspiratory neurons (I-neurons).Methods Brainstem slices from newborn SD rats(0～3d)were made according to the method of Suzue,et al.These preparations included the medial region of the nucleus retrofacialis (mNRF)with the hypoglossal nerve rootlets retained.Respiratory rhythmical discharge activities (RRDA)of BE-neurons/I-neurons in mNRF and activities of the hypoglossal nerve(Ⅻ nerve)were simultaneously recorded by using extracellular microelectrodes and suction electrode,respectively.The effects of 5-HT1A receptors on the respiratory rhythm were investigated by application of 5-HT1A receptor specific agonist 8-OH-DPAT and its specific antagonist PMPPI in the perfusion solution.Results 8-OHDPAT induced a significant increase in the respiratory cycles(RC)and the expiratory time (TE) as well as a decrease in the integral amplitude(IA) and the peak discharge frequency (PFn) of RRDA of BE-neurons and I-neurons. By contrast, PMPPI produced opposite effects on the BE-neurons and I-neurons except that PMPPI changed IA and PFn of BE-neurons/I-neurons nonsignificantly. Conclusion 5-HT1A receptors are involved in the phase-switching between expiration and inspiration by affecting BE-neurons;5-HT1A receptors are also possibly involved in the modulation of rhythmical respiration through the inhibitory synaptic input from I-neurons.