Infusions of 60 pmol/h atriopeptin II into the lateral ventricles stimulated urine flow in both normally hydrated and sodium depleted conscious rats without any increase in the excretion of sodium, potassium or total solutes. The urinary sodium concentration and osmolality both declined significantly. In addition to the renal effects, cerebroventricular infusions of 60 pmol/h atropeptin II reduced salt appetite in rats following depletion of sodium by combined treatment with furosemide diuresis and low sodium diet. Neither 60 nor 600 pmol/h doses had any effect on urine volume or concentration during intravenous infusions, so the effects appear to be mediated by the central nervous system (CNS). Atriopeptins may act through the CNS to facilitate reduction of extracellular volume in synergy with peripheral natriuresis: by increasing urine flow independently from natriuresis; and by restricting further expansion of extracellular volume by reducing salt appetite.