Antacid capacity of two aluminium containing-antacid drugs was evaluated in vitro; the first drug contained aluminium hydroxide (Alternagel), the second, aluminium phosphate (Phosphalugel). The antacid evaluation was performed 1) in a closed system by measuring antacid activity by down titration, 2) by a dynamic evaluation simulating acid secretion and gastric emptying. The results were reported both to the recommended therapeutical dose and to 100 mg aluminium. In static conditions, without gastric emptying, it was shown that aluminium hydroxide and phosphate acted by their buffer capacity in pH range less than or equal to pH 1.5. The therapeutical dose of aluminium phosphate displayed greater antacid activity than aluminium hydroxide, this fact being due to the empiric choice of the doses. With regard to aluminium content, aluminium phosphate activity remained greater than that of aluminium hydroxide although the difference decreased with decreasing pH values. The antacid capacities were related to the emptying outputs. Antacid activity corresponding to 100 mg aluminium was similar in both antacids less than pH 1.5. This effect was dependent on emptying rates. It can be suggested that Al was responsible for antacid activity in both preparations, and that the buffering capacity was supported by the change of aluminium cation in hydrolysis intermediary compounds.