Peptides derived from pro-dynorphin and pro-vasopressin precursors coexist within neurosecretory vesicles of magnocellular neurons in the rat hypothalamus projecting to the posterior pituitary. The secretory activity of these neurons can be stimulated using physiological manipulations known to increase plasma vasopressin levels, such as dehydration and salt-loading. With chronic osmotic challenge, the mRNAs for both pro-dynorphin and pro-vasopressin increase in parallel in the supraoptic and paraventricular nuclei of the hypothalamus, but not within the nonmagnocellular suprachiasmatic nucleus cell groups projecting elsewhere than the neural lobe. The results indicate an example of coordinate regulation of mRNA expression for coexisting peptides within the brain.