It was reported that substance P (SP) plays important role in pain modulation and acupuncture analgesia of nucleus raphe magnus (NRM). The aim of the present study is to reveal ultrastructural morphological features of SP neurons and their synaptic relations with other neural elements in the NRM. The results showed that SP-like immunoreactive (SPLI) neurons are fusiform, oval, or multipolar cells which evenly distributed over the nucleus. At the ultrastructural level, immunostaining was present in the cytoplasmic