Serum concentrations of Theophylline were measured in patients with reversible airways-obstruction during oral and intravenous treatment. The comparison of two sustained-release preparations of Aminophylline (Euphyllin retard 350 mg, Mundiphyllin retard 225 mg), administered according to a randomised study, shows no significant difference. Plasma levels from 5 to 20 mg/l were measured after five days of treatment with either preparation given in average dosage of 10 mg Theophylline per kg body weight. Similar plasma levels, which are associated with therapeutic effects, were measured after the intravenous application of the same dose of Theophylline over 24 hours.