The gastric acid response to insulin hypoglycemia represents an effect of several stimulatory and inhibitory mechanisms. In the intact stomach the direct vagal excitation of the acid secreting glands is the predominant mechanism. After vagotomy, however, the balance between the stimulatory and inhibitory mechanisms is unpredictable. Some stimulatory and inhibitory mechanisms are non-vagal, and may after vagotomy result in a false conception of remaining vagal fibers and complete vagotomy, respectively. Sham feeding may be a safer and more reliable test of completeness of vagotomy. A study of the spontaneous variation of basal acid secretion over several hours after vagotomy in 22 patients showed in 15-min samples a maximal range of 0.49 and 0.65 mmol with a P value of 0.05 and 0.01, respectively. A higher range may thus indicate a true acid response to a given stimulus. In 3 patients with an acid response to sham feeding but no acid response to insulin in a dose of 0.2 IU/kg after vagotomy, a repeated test with a lower dose of insulin resulted in an unequivocal acid response. In 4 patients with no acid response to sham feeding and a substantial acid response to insulin after vagotomy, the acid response to insulin was abolished after pretreatment with an adrenergic blocker. The insulin test may thus give false negative and positive information about the completeness of vagotomy.