We have developed a noninvasive test for gastric acid secretion based on the reaction of ingested magnesium metal with gastric acid to produce hydrogen gas, which is excreted in exhaled air and belches. This test was compared to the standard intubation test by performing both, on different days, in 36 adult Peruvian outpatients referred for gastric analysis; the correlation coefficient for this comparison was 0.71. The new test was repeated in nine subjects, and the resulting test-retest correlation coefficient was 0.83. The new test thus compares favorably with the standard intubation test. Because it is also noninvasive and should therefore be more acceptable to subjects, this new test may be useful for clinical evaluation and research studies on subjects in whom intubation would be difficult or unacceptable. © 1987 Plenum Publishing Corporation.