Nine adult asthmatics with a history of nocturnal symptoms and with morning dips in peak expiratory flow (PEF) were treated for 10-14 days with 24-h controlled-release preparation of theophylline (Th), or a controlled-release preparation of terbutaline (Te), in a double-blind cross-over experiment. During treatment with 450-900 mg Th in the evening morning, plasma drug levels ranged from 53-95 (mean 73) mumol/l. The Te dose was 7.5 mg twice daily. Morning PEF values during Th (mean 338 l.min-1) and Te (316 l.min-1) were not significantly different. There were no significant differences between the treatments in average nocturnal oximetric O2 saturation (91.9% during Th and 91.0% during Te), or the amount of nocturnal body movement, recorded with a static charge sensitive bed (total number of movements 146 during Th and 120 during Te). No difference between the treatments was seen with respect to assessment by the subjects of sleep quality, which was considered fair or good. The findings suggest that in moderately severe asthma, nocturnal oxygenation and sleep quality were similar during the two treatments.