Absorption of theophylline from one commercial product labelled as aminophylline sustained release was compared to the absorption from an oral solution of aminophylline in a single-dose bioavailability study. Aminomal-R tablets had bioavailability (101.2 +/- 19) statistically indistinguishable from that of the standard but showed significantly slower absorption (peak times of 3.6 +/- 1.1 h vs 1.3 +/- 0.8 h) and lower peak plasma concentrations (16.8 +/- 4.7 mg/l/1 g aminoph. dose vs 21.1 +/- 4.2 mg/l/1 g aminoph. dose). Projections of plasma concentrations upon multiple dosing were made from single dose data: the dosage interval (every 12 h) concentration ratio which reflects both the frequency of dosing and the entry of the drug into and removal from the body was of 1.8 vs 3.1.