Chromogranin A (CGA) is a calcium-binding glycoprotein thought to be the precursor of several peptides with defined biological activity. Chromogranin A has been localized in most endocrine cells and many neurons in the CNS. Here we studied its expression in neurons of the hypothalamo-neurohypophysial system, which secrete the neurohormones oxytocin and vasopressin. Light and electron microscopic immunocytochemistry and immunoblot analysis with antibodies specific for CGA revealed high levels of chromogranin A immunoreactivity throughout the hypothalamo-neurohypophysial system. In the supraoptic and paraventricular nuclei, it was characterized by intracytoplasmic labelling of magnocellular somata and processes and of certain astrocytes. Extensive labelling of fibres and dilatations characterized the internal layer of the median eminence and the neurohypophysis, transit and terminal site of the neurosecretory axons, respectively. Tanycyte-like cells in the median eminence also displayed reaction. Simultaneous immunofluorescence showed that oxytocinergic and vaso-pressinergic neurons contain chromogranin A. Electron microscopy revealed that chromogranin A immunoreactivity (visualized by pre-embedding immunoperoxidase or silver-enhanced colloidal gold techniques) was associated with neuro-secretory granules in hypothalamo-neurohypophysial system neurons. In astrocytes and pituicytes, it was seen over the cytoplasm and glial filaments. In tissue from colchicine-treated or immobilization-stressed rats, it was clear that chromogranin A immunoreactivity in the hypothalamus was confined to the hypothalamo-neurohypophysial system. In rats in which neurohypophysial secretion was strongly stimulated by dehydration, immunocytochemistry showed that hypothalamo-neurohypophysial system immunoreactivity significantly increased in the magnocellular nuclei but decreased in the neurohypophysis. On the other hand, chromogranin A distribution was not markedly affected by stress or lactation. These observations demonstrate that chromogranin A is present in neurons and, to a lesser degree, glial cells of the hypothalamo-neurohypophysial system and that its expression is closely related to that of the neurohypophysial peptides.