Healthy adult subjects were given oral sustained-release (SR) aminophylline tablets 225 mg 12-hourly (n = 12) or 450 mg 12-hourly (n = 6) for 5 doses, and all were given aminophylline 225 mg intravenously on a separate occasion to determine individual kinetic parameters. The mean plasma theophylline half-life ( 1 1/2) from the intravenous study was 5.8 hr (SD +/- 1.9). During the 12 hr after the fifth dose of SR aminophylline, the maximum and minimum mean (SD) plasma theophylline levels were 9.7 (3.2) mug/ml and 4.9 (2.0) mug/ml for the 225-mg dose, and 21.1 (2.2) mug/ml and 11.6 (4.4) mug/ml for the 450-mg dose. Side effects were generally mild in the low-dose study, but in the high-dose study 3 subjects reported headache and 1 was withdrawn after 3 doses. Two subjects also noted nausea, and 3 reported insomnia and anxiety in the high-dose study. It was concluded that there was a considerable range of doses needed to achieve adequate plasma levels, and that the formulation studied was capable of producing reasonably stable and satisfactory plasma theophylline levels once individual dose requirements had been established.