Abstract: The present invention provides a composition for measuring the gastric-emptying function and a method for measuring the gastric-emptying function effectively usable in diagnosing dyspepsia. The present invention also provides a dyspepsia diagnostic agent, a method for diagnosing dyspepsia, and a method for measuring the treatment effect on a patient suffering from gastrointestinal disorders caused by insufficiencies in the gastric-emptying function (including a pharmacotherapy). One of the main features of the composition for measuring the gastric-emptying function of the present invention is that a pyrimidine compound, which is converted to a labeled CO2 gas in the body and excreted in expired air, labeled with an isotope of C or O, is used as an active ingredient. The method of the present invention can be conducted by orally administering the composition to a subject, and measuring the amount or behavior of the labeled CO2 excreted in expired air.