Atriopeptin, the atrial natriuretic peptide, is a circulating hormone that is released from the atria of mammalian hearts in response to volume expansion and acts upon the kidneys, adrenal glands and vasculature to regulate fluid and electrolyte homeostasis. Atriopeptin is also present in the brain of the rat. Atriopeptin immunoreactive cell bodies and fibres are found in many areas known to be involved in the central regulation of the cardiovascular system, suggesting that it may be a neuromediator in the central control of fluid and electrolyte balance. The paraventricular nucleus of the hypothalamus, which contains the cell bodies of neurons that secrete vasopressin from the posterior pituitary gland, receives a dense innervation from atriopeptin-like immunoreactive fibres. We have studied the effect of atriopeptin on the electrical activity of single neurons in the paraventricular nucleus of anaesthetized rats and found that atriopeptin is a potent inhibitor of putative vasopressin neurons. Atriopeptin, which has systemic actions that oppose those of vasopressin, may act as a neuromodulator in the brain to prevent vasopressin secretion.