The peak acid output to insulin 10-14 days after proximal gastric vagotomy (PGV) for uncomplicated duodenal ulcer in a control group of 11 men was 1.5 mmol/h, and this rose significantly (P less than 0.01) after a mean interval of 26.1 months to 5.3 mmol/h. The rise in the PAOI in 7 male patients undergoing PGV and hepatic interposition for duodenal ulcer was from 1.0 mmol/h to 4.5 mmol/h (P less than 0.05) over a mean interval of 13.1 months. No significant difference between the changes in the PAOI in the two groups of patients could be detected. It is concluded that separation of the lesser curve of the stomach from the cut edge of the lesser omentum by the left lobe of the liver after PGV in an attempt to block vagal regeneration does not alter the postoperative rise in acid secretion.