The present study was performed on 22 cats to explore whether r-aminobutyric acid(GABA)

endogenous opioid peptide and substance P(SP)were involved in the regulation ofpresynaptic inhibition in acupuncture analgesia.The size of the antidromic compound C action po-tentials of the sural nerve evoked by the testing stimulation in spinal cord was measured as an in-dicator of C-afferent terminal excitability.It was found that electro-acupuncture(EA)at“Huan-tiao”(GB30)and“Yang lingquan”(GB34)points induced significant enlargement of the an-tidromic C-waves

showing the depolarization of prsynaptic terminals of primary C-afferents wasenhanced.The depolarization effect of EA was significantly reduced by bicuculline

naloxone andthe antiserum of SP locally applied to the surface of spinal cord respectively.It is supposed thatGABA

endogenous opioid peptide and SP may be involved in the regulation of presynaptic inhibi-tion in acupuncture analgesia.