The mechanism by which magnesium acts on gastric acid secretion is obscure. It has been demonstrated that magnesium inhibits basal as well as hypercalcemic-induced acid secretion. It has furthermore been shown that magnesium does not interfere with gastrin release. It is not, however, known if magnesium acts as an antagonist to gastrin. To elucidate this the combined effect of pentagastrin (0.5 mug/kg-hr) and magnesium sulphate (0.4 meq Mg++/kg-hr) was studied in 10 healthy subjects. The secretory plateau obtained during pentagastrin infusion was unchanged during the combined infusion of pentagastrin and magnesium. This applies to volume secretion, acidity as well as acid output.