The absolute bioavailability of theophylline was investigated by comparing the areas under concentration-time curves for intravenous theophylline with a plain uncoated anhydrous theophylline tablet and a theophylline solution. Twenty asthmatic adults received approximately 7.5 mg/kg theophylline intravenously over 30 minutes; either seven days before or after the i.v. dose, 10 of these patients received tablets and the remainder solution in a similar dose. Blood samples were obtained at 0, 10, 20, 30, 45, 60, 90, 120, 180 and 240 minutes and then every two hours for at least 12 hours. Theophylline concentration was measured in serum by high-pressure cation exchange chromatography. The fraction of the dose absorbed averaged 0.96 +/- 0.03 for the tablet while the value for the solution was 0.99 +/- 0.02. The time of peak absorption averaged 2 +/- 0.3 hours for the tablets and 1.4 +/- 0.3 hours for the solution. The maximum serum concentration attained was 15.3 +/- 0.7 microng/ml after a dose of 7.6 +/- 0.4 mg/kg of the tablet, and 14.6 +/- 0.6 microng/ml after a dose of 7.3 +/- 0.2 mg/kg of the solution. Absorption of the tested theophylline tablets and solution approached 100% of the available drug.