Abstract: The ability of magnessium trisilicate to adsorb tetracycline find its relation to pH of the medium have been investigated UV-spectrophotometrically. The effect of magnesium trisilicate on absorbing tetracycline and its effect in the presence of some surfactants and citric acid in vivo have also been studied by using the data of urinary excretion. The relative bioavailability of three different tetracycline suspensions has been determined and the parameters have been statistically compared simultaneously. The results have shown that, the adsorptive capacity of magnesium trisilicate is enhanced,as the pH is raised. Sodium lauryl sulfate and citric acid have exhibited a significant desorption on magnesium silicate in vitro, but Brij-35 hasn't. The simultaneous oral administration of tetracyclline and magnesium trisililcate can markedly reduce the absorption of the former (p