This study was performed to investigate the effect of prolonged (6-hour) infusions of salmon calcitonin on basal gastric acid secretion and serum gastrin and calcium levels in 9 normal volunteers. Three tests, during which gastric acid secretion was measured continuously for 8 hours, were performed on each subject. After 1 hour, calcitonin 2,5 or 5,0 micrograms/h or a saline placebo (in random order) was infused intravenously for 6 hours. Calcitonin significantly inhibited basal gastric acid secretion, the effect reaching statistical significance (P less than 0,05) after 90 minutes with the larger dose and after 150 minutes with the smaller dose. Eight of the 9 tests using the larger dose and 6 of the 9 using the smaller dose rendered the subjects achlorhydric. The changes in acid secretion occurred independently of a slight drop in serum calcium levels. No changes in blood gastrin levels occurred. Calcitonin is capable of completely abolishing gastric acid secretion when infused over a long period.