This study investigated the involvement of serotonergic mechanisms of the lateral parabrachial nucleus (LPBN) in the control of sodium (Na⁺) excretion, potassium (K⁺) excretion and urinary volume in unanesthetized rats subjected to acute isotonic blood volume expansion (0.15 M NaCl, 2 ml/100 g of body weight over 1 min) or control rats. Plasma oxytocin (OT), vasopressin (VP), and atrial natriuretic peptide (ANP) levels were also determined in the same protocol. Male Wistar rats with stainless steel cannulas implanted bilaterally into the LPBN were used. In rats treated with vehicle in the LPBN, blood volume expansion increased urinary volume, Na⁺ and K⁺ excretion and also plasma ANP and OT. Bilateral injections of serotonergic receptor antagonist methysergide (1 or 4 µg/200 l) into the LPBN reduced the effects of blood volume expansion on increased Na⁺ and K⁺ excretion and urinary volume, while LPBN injections of serotonergic 5HT_2a/HT_2c receptor agonist, 2.5-dimetoxi-4-iodoamphetamine hydrobromide (DOI, 1 or 5 µg/200 l), enhanced the effects of blood volume expansion on Na⁺ and K⁺ excretion and urinary volume. Methysergide (4 µg) into the LPBN decreased the effects of blood volume expansion on plasma ANP and OT, while DOI (5 µg) increased them. The present results suggest the involvement of LPBN serotonergic mechanisms in the regulation of urinary sodium, potassium and water excretion and hormonal responses to acute isotonic blood volume expansion.