The bioavailability of theophylline from single doses of an elixir (Elixophyllin) and two different tablet formulations, as compared to intravenous aminophylline, was studied with a crossover design in 12 normal volunteers. Both tablet formulations (Marax and Tedral) contain ephedrine. Marax contains hydroxyzine hydrochloride, and Tedral contains phenobarbital. The absorption of theophylline was most rapid from the elixir, whereas that from Marax was faster than that from Tedral. The peak concentrations of theophylline after administration of the 3 oral dosage forms were in the order, elixir greater than Marax greater than Tedral, however, the time to achieve peak concentration was highly variable and did not differ significantly among the 3 products. On the basis of area under the serum concentration-time curves, the absorption of theophylline from the elixir and from Marax was essentially complete. The area under the serum concentration curve after administration of Tedral was significantly less than that after intravenous aminophylline, elixir, and Marax; however, when the individual areas under the concentration curves were adjusted for intrasubject variation in elimination rate constant, the mean area under the concentration curve after Tedral no longer differed significantly from those of the intravenous and other two oral products. A large degree of intersubject variation in the oral absorption of theophylline was observe din this study. Therefore, in addition to the well-documented, large individual variation in the serum clearance of theophylline, intersubject differences in the absorption of the drug is another factor that complicates proper adjustment of the dose in oral theophylline therapy.