The reaction rates, duration times and neutralizing capacities of the antacids which are frequently used in Korean market and three different commercial combination products were evaluated in vitro by Fuchs method and Johnson-duncan method, respectively. In vivo tests of combination products were determined in the fasted state of rat by Aspiration method. Comparing the result of in vitro test with that of in vivo test, the maximal pH was lowered by 2-3 value and the durational time increased by two folds in vivo test. Each antacid composition and combination products from three phamaceutical companies (A, B, and C) were studied, respectively. The duration times measured by Fuchs method were double compared to those by Johnson-Duncan method. A and C preparation maintained the pH range from 3 to 7 for 60 min by Fuchs method. In vovo test, maximum pH of A, B and C preparation was 6.50, 3.65, 2.65 and duration time of those was 200, 500, 0 min, respectively.