12 Wistar rats (220-250g) were divided into control and experimental groups. Pain threshold was determined by WQ-9E Pain Threshold Measuring Apparatus. EA was applied at bilateral "Zusanli" points. The animals showing EA analgesia and the control animals were sacrificed and the ventrolateral part of PAG and the NRM were taken out for electron microscopic observation. The peripheral region of ventrolateral PAG (PAG-PR) and the NRM contained a wide variety of cell types and synaptic relationships. The neurons and axo-dendritic synapses of the PAG-PR and NRM were analysed in EA and control groups. In EA analgesia group, the clear round vesicle-containing presynaptic boutons and the presynaptic bouton with clear round vesicles and granular vesicles showed a significant decrease of their vesicle content. Part of the mitochondria and endoplasmic reticulum in the neurons appeared the expanded profiles. These ultrastructural changes of the PAG and the NRM during EA analgesia may indicate an increased release of acetylcholine, biogenic amines and/or peptide neurotransmitters and that the neurons were in active functional state.