In a double-blind cross-over study, the effects of slow release (S-R) terbutaline tablets (b.i.d. 0.25 mg/kg per day) and S-R theophylline (5.31 mg/kg morning and 10.62 mg/kg evening) were compared in eleven patients with nocturnal asthma. On day seven of each treatment period, drug serum concentrations and peak expiratory flow (PEF) were measured every 2h over a 24-h period. During daytime, terbutaline concentrations ranged from 1.6-14.1 (median 4.5) microgram/l and during the night from 2.1-18.7 (median 4.9) micron/l. Theophylline concentrations ranged from 3.9-24.3 (median 11.5) mg/l during the day and from 3.3-20.9 (median 10.4) mg/l at night. Nocturnal wheezing occurred during theophylline treatment in four patients 7 times and during terbutaline treatment in six patients 22 times. Daytime PEF values were 472 +/- 161 l/min during theophylline therapy versus 445 +/- 169 l/min during terbutaline therapy (p less than 0.05). In the night and early morning there was no significant difference between PEF values with the two treatment forms. During theophylline treatment, fewer inhalations of beta 2-sympathomimetics were used, and there were fewer side effects. One patient experienced severe asthmatic attacks during the terbutaline treatment period. The patients preferred theophylline for the treatment of nocturnal asthma.