The discrete anatomical distribution of arginine vasopressin and corticotropin releasing factor (CRF) immunoreactivity in the paraventricular nucleus (PVN) of the rat hypothalamus is altered after adrenalectomy. Not only is the immunostaining of both peptides enhanced, but vasopressin immunoreactivity, normally confined to the magnocellular subdivision, becomes clear in a large percentage of CRF neurones in the parvocellular subdivision. These changes in immunoreactivity may reflect changes in post-translational events, peptide metabolism or genomic activity that lead indirectly or directly to the enhanced expression of vasopressin. Here we report that levels of transcripts homologous to vasopressin messenger RNA increase in the PVN after adrenalectomy, in parallel with increases in vasopressin immunoreactivity. In fact, after adrenalectomy, vasopressin mRNA can be detected in CRF-immunoreactive neurones. These results indicate that a considerable degree of plasticity is retained by the adult neuronal genome of the rat and that this plasticity may be modulated by the endocrine environment.