A Golgi-Cox study of the supraoptic nucleus of the adult rabbit has revealed the presence of three major cell types: (1) large multipolar neurons; (2) large bipolar neurons, and (3) small interneurons. The multipolar and bipolar neurons demonstrated regional variation in the rostral and caudal aspects of this nucleus, while the interneurons appeared identical throughout the nucleus. This study provides detailed cytoarchitectural descriptions of the magnocellular neurosecretory cells in the supraoptic nucleus and establishes the presence of an interneuronal cell population through which incoming afferent messages could influence the neurosecretory neurons.