In situ hybridization has been used to identify specific hypothalamic magnocellular neurons, in normal and Brattleboro rats, that contain vasopressin (VP) or oxytocin (OT) mRNA. The subnuclear distribution of identified neurons in hypothalamic magnocellular nuclei after hybridization with several probes specific for OT or VP mRNA was in direct agreement with immunocytochemical descriptions of the distribution of cells containing VP or OT hormone. The number of grains per cell suggested that Brattleboro rats contained greater levels of OT mRNA, while hybridization with VP probes produced fewer grains in tissue from Brattleboro rats compared to normal rats. This paper provides the first cell-by-cell description of VP gene expression in the Brattleboro rat and demonstrates that VP gene transcription is confined precisely to the magnocellular neurons that normally synthesize VP hormone in normal rats.