The atrial appendages contain most of the atrial natriuretic factor (ANF) in the mammalian heart, and atrial appendage mechanical function predicts ANF secretion during volume loading. To demonstrate the crucial role of the atrial appendages in ANF release, we first measured hemodynamics and changes in plasma ANF after injection of 1,000 ml i.v. normal saline in conscious dogs and again after bilateral atrial appendectomy; we next measured changes in renal function using infusions of atriopeptin 24 to achieve plasma levels corresponding to levels achieved during volume loading; and we lastly measured renal function during acute volume expansion and also after atrial appendectomy. Plasma ANF increased from 65±11 to 246±54 pg/ml after volume loading but did not increase after atrial appendectomy. Atrial appendectomy did not alter the tachycardia or hemodynamic effects of volume loading. Infusion of 10 ng/kg/min atriopeptin 24 increased plasma ANF from 50±9 to 234±54 pg/ml, increased urine output 34±10%, and increased sodium excretion 62±10% in dogs with intact atrial appendages. Renal function was compared in dogs before atrial appendectomy: 20, 40, and 60 minutes after volume loading, urine flow rate increased by 5.9±0.5, 6.9±0.4, and 4.4±0.8 ml/min, while sodium excretion increased by 717±60, 839±84, and 582±57 μeq/min. After atrial appendectomy urine flow rate increased 2.1±0.7,2.7±0.7, and 2.0±0.6 ml/min, and sodium excretion increased only by 327±110, 324±77, and 340±92 μeq/min (p<0.01) during volume loading. The total fraction of water and sodium, 63±5% and 60±4%, respectively, of the load excreted in 4 hours was significantly reduced to 30±5% and 35±3% of the load (p<0.01) after bilateral atrial appendectomy. We conclude that bilateral atrial appendectomy eliminates ANF release and blunts renal excretion of sodium and water after a large acute volume load in conscious dogs.