&NA;This study investigated whether excision of either the right or left atrial appendage of rats alters their natriuretic response and the release of atrial natriuretic factor during acute blood volume expansion or reduction. These animals were subjected to a thoracotomy and either had their right or left atrial appendages removed or underwent a right or left atrial sham appendectomy for comparative, control purposes. Intrajugular vein, intracarotid artery, and intravesical catheters were installed 3‐4 weeks later under sodium pentobarbital anesthesia. Then, when the rats were conscious, blood volume was expanded using blood from donor rats once every 15 minutes in 3 increments of 10% of the calculated total blood volume at a rate of 5 ml/kg/min. Blood and urine samples were collected before volume expansion and at the end of each 15‐minute period, with the withdrawn blood being replaced. A maximal fourfold increase in urinary volume, urinary sodium excretion, and plasma atrial natriuretic factor was observed in all but the right‐atrial‐appendectomized animals. Plasma atrial natriuretic factor, urinary volume, and urinary sodium excretion were correlated in all 4 groups. No significant changes in blood pressure or hematocrit were noted. Plasma vasopressin, measured at the end of volume expansion, was significantly lower in animals subjected to left atrial appendectomy. Highperformance liquid chromatography of plasma from the control groups indicated that most of the released ANF during blood volume expansion corresponded to a high molecular weight peptide. Additional rats, processed as above, were subjected to 10% blood volume decrements. Urine and blood were collected before the blood volume reduction and at 15‐minute intervals after each of the 3 decrements of 10% volume. A gradual and marked decline in urinary volume and sodium excretion was seen in all 4 groups but only the control groups showed a significant lowering of plasma atrial natriuretic factor. Plasma vasopressin was equally high in all groups. A significant but negative correlation was evident between the plasma levels of atrial natriuretic factor and vasopressin (r= −0.51,p<0.01). Left atrial appendectomy induced a chronic elevation of systemic arterial blood pressure, which may represent a new model of experimental hypertension whose mechanisms remain to be investigated. It can be concluded that the right atrium, probably by releasing atrial natriuretic factor, may be involved in the short‐term regulation of blood volume, but other neural and hormonal systems (such as vasopressin) may play more important roles in volume conservation during hemorrhage. (Circulation Research1987;61:99‐106)