The presence of volume receptors and a potent natriuretic factor (ANF) in mammalian cardiac atria strongly suggests a central role of the atria in extracellular fluid volume regulation. ANF is stored within granules in atrial appendages, and their removal could alter the response to volume overload. We tested this hypothesis in conscious Wistar rats two weeks after sham operative or atrial appendectomy. The results indicate that removal of the atrial appendages significantly reduced their urinary excretion of water, sodium and potassium during the first hour following acute volume overload. It is concluded that atrial appendectomy alters the ability of rats to handle acute volume overload possibly through a reduction in the ANF available for release.