<b><i>Introduction:</i></b> Water homoeostasis is achieved by secretion of the peptide hormones arginine vasopressin (AVP) and oxytocin (OXT) that are synthesized by separate populations of magnocellular neurones (MCNs) in the supraoptic and paraventricular (PVN) nuclei of the hypothalamus. To further understand the molecular mechanisms that facilitate biosynthesis of AVP and OXT by MCNs, we have explored the spatiotemporal dynamic, both mRNA and protein expression, of two genes identified by our group as being important components of the osmotic defence response: <i>Caprin2</i> and <i>Creb3l1</i>. <b><i>Methods:</i></b> By RNA in situ hybridization and immunohistochemistry, we have characterized the expression of <i>Caprin2</i> and <i>Creb3l1</i> in MCNs in the basal state, in response to dehydration, and during rehydration in the rat. <b><i>Results:</i></b> We found that <i>Caprin2</i> and <i>Creb3l1</i> are expressed in AVP and OXT MCNs and in response to dehydration expression increases in both MCN populations. Protein levels mirror the increase in transcript levels for both CREB3L1 and CAPRIN2. In view of increased CREB3L1 and CAPRIN2 expression in OXT neurones by dehydration, we explored OXT-specific functions for these genes. By luciferase assays, we demonstrate that CREB3L1 may be a transcription factor regulating <i>Oxt</i> gene expression. By RNA immunoprecipitation assays and Northern blot analysis of <i>Oxt</i> mRNA poly(A) tails, we have found that CAPRIN2 binds to <i>Oxt</i> mRNA and regulates its poly(A) tail length. Moreover, in response to dehydration, <i>Caprin2</i> mRNA is subjected to nuclear retention, possibly to regulate <i>Caprin2</i> mRNA availability in the cytoplasm. <b><i>Conclusion:</i></b> The exploration of the spatiotemporal dynamics of <i>Creb3l1</i>- and <i>Caprin2</i>-encoded mRNAs and proteins has provided novel insights beyond the AVP-ergic system, revealing novel OXT-ergic system roles of these genes in the osmotic defence response.