Gastrin is released by food rich in proteins and by vagal mechanisms. HCI and possibly secretin and glucagon inhibit gastrin release. In the wide range of actions of gastrin, stimulation of gastric acid secretion is the most important. With the advent of radioimmunochemical methods for the determination of gastrinaemia, it has been shown that gastrin exists in a number of forms of different molecular weight. To estimate the validity of gastrin radioimmunoassay it is necessary to demonstrate that decrease in antibody-bound labelled antigen is unrelated to non-specific interference by unknown substances present in serum samples, and that the antiserum reacts with endogenous hormone in an identical manner. Heterogeneity of gastrin in serum may affect the validity of the radioimmunoassay. Hypergastrinaemia associated with hyper-normochlorhydria occures in gastrinoma, hyperplasia of antral gastrin cells, diseases with delayed gastric emptying, retained antrum, short bowel syndrome,renal failure. Hypergastrinaemia associated with hypo-achlorhydria occurs in atrophic gastritis without extensive antral lesion and after vagotomy. Gastrin radioimmunoassay can be used for the mass screening of subjects with atrophic gastritis, a high risk group for gastric cancer.