The effects of substance p (sp) microiontophoretically applied on spi- nal dorsal horn neurones were observed in 32 rats. Sp caused an increas in excitability for majority of the units affected by sp and a decreas only for a few units. The examination in receptive character of 25 units showed that 20 of 25 units can be excited by noxious mechanical stimulus or/and noxious heat or/and bradykinin applied and 18 of 20 units can also be excited by micro- iontophoretic sp. But the rest which were non-nociceptive units failed to respond to sp or were slightly inhibited by sp. The results support the po- int of view that sp acts as a neurotransmitter or modulater in spinal pain pathways. The excitatory effect of 15 of 17 units caused by sp was inhibited by electro-acupuncture stimulation at Zusanli points. 4 of 5 units tested for the effect of naloxone on the electro-acupuncture action showed that the i- nhibitory action of acupuncture stimulation on excitatory effect of sp fai- led to be blocked by microiontophoretic naloxone. It is suggested that the mechanism of acupuncture analgseia in spinal cord includs postsynaptic nhi- bition and endogenous opiate-like substance may not play an important role in the inhibitory mechanism.