Thiazolo[3, 2-a]thieno[3, 2-d]-, [3, 4-d]- and [2, 3-d]pyrimidin-5-one derivatives (6, 11, and 16), and polymethylene condensed thieno[3, 2-d]-, [3, 4-d]- and [2, 3-d]pyrimidin-5-one derivatives (19-21), in which the oxygen atom of the oxazolidine moiety in 3 was replaced by a sulfur atom or methylene groups, were synthesized and evaluated for gastric antisecretory activity in pylorus-ligated rats. The structure-activity relationships of these compounds are discussed.