A battery of tests was applied to 60 patients at periods varying from 2 to 24 months after they had undergone truncal vagotomy (42 with pyloroplasty, 18 with gastro-enterostomy). The major comparison was between those with endoscopic evidence of recurrent duodenal ulcers, and those in whom endoscopy showed no abnormality. Tests which differentiated between the two groups to a statistically significant extent were those for increased acid concentration in the first hour after insulin and serum gastrin, which was lower in this group. Useful, but statistically not significant tests, in patients with no endoscopic evidence of recurrent ulcers, were a postoperative decrease in basal acid output and maximal acid output after pentagastrin or insulin. Symptoms, the combined pentagastrin-insulin test, the volume of acid secreted after insulin, and the acid concentration in the second hour after insulin stimulation were poor indicators for differentiating between patients with recurrent ulcers and those without ulcers. Almost as much information was obtained by the postoperative basal acid output alone as by the pentagastrin and insulin stimulatory tests.