SUMMARY Insulin-stimulated gastric secretion alone, without reference to basal secretion, has been examined in 45 male patients with duodenal ulcer in whom no gastric operation had been performed and in 124 patients following vagotomy for duodenal ulcer. Gastric juice was examined in terms not only of conventional indices, observed volume, titratable acidity and acid output, but also VG, the volume corrected for pyloric loss and duodenal reflux.The range of secretion of the unoperated subjects was established in terms of peak and half-to two-hour values for all indices. By reference to these ranges, secretion of postvagotomy subjects could be divided into two groups: (a) those with secretion within the preoperative range, and (b) those with secretion less than the lower limit of the preoperative range.The best discrimination was given by VG; those within the preoperative range (peak VG in excess of 140 ml/hour and VG half to two hours in excess of 105 ml/hour) had a 50% liability to recurrent ulcer, while those below the preoperative range had a zero liability to recurrent ulcer. Of the conventional indices acid output gave the best discrimination, which was almost as good as VG. Peak acid output of 8 mmol/hour or acid output one half to two hours of 5 25 mmol/hour discriminated into two groups, with a 50% or zero liability to recurrent ulcer. Titratable acidity (Hollander's index of secretion), being highly susceptible to reflux, was not an adequate discriminant.Surgical division of the vagus nerves as a treatment for duodenal ulceration was reintroduced by Dragstedt and Owens in 1943. After reporting preliminary results on canine stomach pouches (Hollander, Jemerin, and Weinstein, 1942) Hollander (1946) described the use of the insulin test for detecting the presence of intact vagal fibres in patients following vagotomy. Two years later (Hollander, 1948) he modified his criteria and defined a positive test as one in which the free acidity in response to insulininduced hypoglycaemia was more than 20 mmol/ litre in excess of basal acidity. However he remained cautious with regard to these criteria saying, 'At no time have we stated that the test can predict the chances that the patient will be relieved of his ulcer or its symptoms, or that they will not recur at some future date'. After another two years' experience he 'Based on a paper read to the British