The ultrastructures of synapses in the head of caudate nuclei of rats were studied by means of electron microscopic technique. The experimental group of rats, in which the pain threshold was elevated after electro-acupuncture, showed no change of the number of the granular vesicles, but a significant decrease of the number of clear synaptic vesicles (without dense core) in the presynaptic boutons. This might be correlated to release of acetylcholine in the synapse.