The effect of magnesium-aluminum hydroxide antacid tablets or liquid on food-stimulated gastric acidity was evaluated in vivo in eight duodenal ulcer patients. Experiments were performed also in vitro. Although in vitro neutralizing capabilities of the two preparations were the same (28.3 +/- 0.1 mmol/liter), the duration of effect of tablets was longer in vivo. For example, at 3 3/4 hr after an eaten meal, mean hydrogen ion concentration with three tablets was 29.8 +/- 9.9 mmol/liter while with 15 ml liquid it was 59.6 +/- 10.3 mmol/liter (P less than 0.05). Additional studies were performed in vitro to explain the longer duration of effect of tablets compared with liquid in vivo. These experiments suggested that tablets reacted more slowly with acid than did liquid and that saliva, which was mixed with tablets during chewing, also contributed to the prolonged duration of effect.