Vasopressin secretion from the magnocellular neurosecretory cells (MNCs) is crucial for body fluid homeostasis. Osmotic regulation of MNC activity involves the concerted modulation of intrinsic mechanosensitive ion channels, taurine release from local astrocytes as well as excitatory inputs derived from osmosensitive forebrain regions. Extracellular signal‐Regulated protein Kinases (ERK) are mitogen‐activated protein kinases that transduce extracellular stimuli into intracellular post‐translational and transcriptional responses, leading to changes in intrinsic neuronal properties and synaptic function. Here, we investigated whether ERK activation (i.e. phosphorylation) plays a role in the functioning of forebrain osmoregulatory networks. We found that within 10 minutes after intraperitoneal injections of hypertonic saline (3 M, 6 M) in rats, many phosphoERK‐immunopositive neurones were observed in osmosensitive forebrain regions including the MNC containing supraoptic nuclei. The intensity of ERK labelling was dose‐dependent. Reciprocally, slow intragastric infusions of water that lower osmolality reduced basal ERK phosphorylation. In the supraoptic nucleus, ERK phosphorylation predominated in vasopressin neurones vs. oxytocin neurones and was absent from astrocytes. Western blot experiments confirmed that phosphoERK expression in the supraoptic nucleus was dose dependent. Intracerebroventricular administration of the ERK phosphorylation inhibitor U 0126 prior to a hyperosmotic challenge reduced the number of both phosphoERK‐immunopositive neurones and Fos expressing neurones in osmosensitive forebrain regions. Blockade of ERK phosphorylation also reduced hypertonically‐induced depolarisation and increase in firing of supraoptic MNCs recorded in vitro. It finally reduced hypertonically‐induced vasopressin release in the bloodstream. Altogether, these findings identify ERK phosphorylation as a new element contributing to the osmoregulatory mechanisms of vasopressin release. This article is protected by copyright. All rights reserved