<正> This study was designed to test whether endogenous opiate peptide-containing interneurons in the spinal cord act in series with long descendingserotonergic fibers or not.The experiments were performed on 21 rabbitsimmobilized with gallamine triethiodide.The nociceptive discharges of 21parafascicular neurons were recorded extracellularly

all of which could bestrongly inhibited by imitative descending inhibitory action produced bystimulating the isolated centrifugal stump of the dorsal half of spinal cord.Afterinjecting cyproheptadine

an antagonist of serotonin receptor

the descendinginhibitory effect of 8 neurons were blocked frofoundly

but in other 13 neuronsthey could not.In 5 neurons

which were not affected by cyproheptadine

naloxone

an antagonist of morphine receptor

administration could block thedescending inhibitory effect.On the contrary

in 6 neurons

which wereinfluenced by cyproheptadine

naloxone failed to block the descending inhibitoryeffect on 5 neurons

except other one.These results further confirm our hypothesisproposed earlier that endogenous opiate peptide-containing interneurons act inseries with long descending noradrenergic fibers in spinal cord to be involved inthe descending antinociceptive actiori

while they might have little

if any

to dowith the descending serotonergic system in this action.