Antacids are often the first therapeutic approach in patients with pyrosis. We carried-out a study on 8 healthy volunteers who underwent a 24 hour gastric pH-metry to assess the real efficacy of magnesium hydroxide administration on gastric acidity. Magnesium hydroxide was alternatively administered at different dosages (400 mg or 800 mg) in the population studied. Our results showed a mild and non reproducible response to lower dose, but, on the contrary, the 800 mg dose always induced an immediate, effective and prolonged antacid action, reaching a maximum pH value of 5 and lasting up to 40 minutes. Our study confirms, using a modern and reliable technique as the 24-hour gastric pH-metry, the antacid activity of magnesium hydroxide.