The effect of a magnesium-aluminum hydroxide antacid (Maalox) on the oral absorption of aminophylline tablets was studied. Twelve healthy adults were administered 200 mg of aminophylline alone or with 30 ml of antacid in a complete crossover study. Blood samples were drawn at 0.33, 0.67, 1, 2, 4, 8, 12, and 24 hours following theophylline (as aminophylline) administration. Theophylline plasma levels were measured by high-performance liquid chromatography. The plasma theophylline concentrations of the group receiving theophylline only were significantly greater than those of the group receiving theophylline plus antacid at the 0.67- and 1-hour sample times only (p less than 0.05). The extent of theophylline absorption and the eliminated rate constant were not significantly affected by the antacid. Antacid significantly decreased theophylline's absorption rate constant (p less than 0.05), indicating a slower absorption of theophylline with antacid. Concurrent administration of the antacid Maalox should not significantly change theophylline's clinical effect.