Human atrial natriuretic factor (ANF) levels were measured before and after peritoneovenous shunt implantation in 10 cirrhotic patients with ascites, in whom sodium retention is a major clinical problem. The mean preoperative plasma level of ANF was 82 ng/L (normal range, 5-80 ng/L). Peritoneovenous shunting resulted in a significant rise in plasma ANF to 308 ng/L (p less than 0.0025) immediately after operation. This was followed by a constant fall until the seventh postoperative day, when mean plasma ANF was still significantly elevated (149 ng/L) compared with the preoperative value (p less than 0.01). Three months after shunt implantation mean plasma ANF had returned to the preoperative level (75 ng/L). Mean sodium excretion increased from 2.6 mEq/h preoperatively to 10.2 mEq/h at the second postoperative day (p less than 0.025). No direct relationship was noted between changes in plasma ANF level and changes in urinary sodium excretion after shunt implantation. These data demonstrate an intact ANF release response to intravascular volume expansion in cirrhotic patients with ascites, but exclude ANF as the diminished natriuretic factor as proposed by the overflow theory of ascites formation. Sodium excretion and fluid retention seem to be the result of vascular underfill and fluid maldistribution, and hormonal changes are likely to be secondary to them.