Serum gastrin has been measured by radioimmunoassay following intravenous secretin in 86 control subjects, 9 patients with pernicious anaemia and 9 patients with Zollinger-Ellison syndrome. In 55 control subjects, Boots secretin (2 U/kg) produced a significant fall in serum gastrin from 40 ± 3.8 to 17 ± 2.3 pg/ml; in 31 control subjects, GIH secretin (1 U/kg) produced a significant fall from 41 ± 4.4 to 14 ± 2.5 pg/ml. In 9 patients with pernicious anaemia, both injection and infusion of Boots secretin caused a significant fall in serum gastrin, 1,030 ± 70 to 360 ± 51 pg/ml and 1,055 ± 70 to 310 ± 30 pg/ml, respectively. By contrast, in 9 patients with Zollinger-Ellison syndrome, Boots secretin caused a substantial rise in serum gastrin in six and no change in the other three. It is suggested that secretin inhibits gastrin release from antral cells and stimulates its release from islet cell tumours; this paradox may have diagnostic application in the Zollinger-Ellison syndrome.