The absolute bioavailability of theophylline in two commercially available preparations of aminophylline, a sustained-release formulation (Euphyllin retard tablets) and a solution (Euphyllin solution), was determined in eight healthy volunteers following oral administration of a single dose. Percent bioavailability was obtained by comparing AUC data for theophylline concentration-time curves measured over 24 h with corresponding data following intravenous doses of theophylline. Absorption of theophylline from solution was rapid and complete. Bioavailability amounted to 111 +/- 16% (X +/- SD) compared to i. v. administration. Absorption from the sustained-release tablet appeared more erratic and less complete: the time of peak theophylline concentration varied between 2 and 14 h. The mean plasma level curve showed a plateau between 2 and 14 h bioavailability was 72 +/- 13%. In addition, the sustained-release formulation was administered twice daily (350 mg) for 4 days. Plasma concentrations during multiple dosing of the sustained release formulation were with one exception below 10 mg/l. On day 4 at steady state, peak theophylline concentrations were 8.1 +/- 0.63 mg/l, trough levels 6.2 +/- 1.4 mg/l in the morning and 5.6 +/- 0.67 mg/l in the evening.