Ultrastructural changes of synapses in the rat substantia gelationsa of the spinal caudal trigeminalnucleus were quantitatively examined following electrical acupuncture at "Sibai" for 30 minutes. Vari-ous structural changes were observed: Spherical synaptic vesicles were clustered toward the presynapticmembrane

the number of the vesicles tended to decrease

the. width of the postsynaptic membrane isincreased and mitochondria were also increased in the bouton. These various types of synaptic alter-ations related to boutons are interpreted as the morphological changes induced by the acupuncture inthe region corresponding to "Sibai".