Twenty-four hour intragastric pH during normal daily activity has been studied by nasogastric intubation and aspiration of gastric samples in seven patients four years after vagotomy (four truncal, three selective). Two of these patients also had pre- and early postoperative studies. Mean pH was inversely correlated with basal, insulin- and pentagastrin-stimulated acid outputs. However, one of four patients after truncal vagotomy with drainage, and two of three after selective vagotomy and drainage had most of their hourly intragastric pH values more acidic than pH 3.5 despite more than 70% reductions in preoperative stimulated-acid outputs.