Gastric acid responses to intragastric administration of graded doses (14-30-45 mg/Kg) of copper (II) complexes with aminoacids (two simple complexes, Cu-Trp2 and Cu-Phe2, and on mixed complex, Cu-Trp-Phe) were investigated in Shay rats. The results (% inhibition of H+ mEq/4h) shown that graded doses of copper complexes caused a progressive inhibition on gastric acid secretion, and that percentage inhibition values were, for all complexes, in linear correlation with employed doses. A possible mechanism by which copper complexes affect gastric acid secretion is discussed.