Serum gastrin levels in patients with pernicious anaemia were measured by immunoassay in the fasting state, following gastric perfusion with 0.9% saline, 0.1N hydrochloric acid, and solutions of increasing acidity, and after the intravenous injection or infusion of secretin. The fasting serum gastrin level was measured in 21 patients with pernicious anaemia and found to be elevated at 1,036 +/- 215 pg per ml. Gastric perfusion with saline (pH 4.7) caused a mean fall in serum gastrin of 30% in four patients; perfusion with hydrochloric acid caused a further slight fall. Perfusion with solutions of increasing acidity resulted in a sharp fall in serum gastrin levels when the acidity was changed from pH 6 to pH 4. A single intravenous injection of secretin produced a mean maximal fall of 44% in the serum gastrin level in four patients, whereas continuous infusion of secretin produced a fall of 35% in four other patients. These studies suggest that the gastrin-secreting cells of the stomach are not affected by the atrophic process in pernicious anaemia and remain subject to the regulating control of acid and secretin.