The effect of terbutaline sulphate in slow-release (SR) tablets (Bricanyl Depot), 5 mg twice daily, was compared with that of terbutaline sulphate in ordinary tablets (Bricanyl), 2.5 mg three times daily, in a double-blind, randomized, cross-over study during 2 consecutive weeks in 10 asthmatic children. Plasma concentrations and urinary excretion of terbutaline were measured at various times during both treatment periods. The SR tablets produced a higher mean plasma concentration in the morning and a smaller peak-trough variation over the day than the ordinary ones. No differences between the two treatments were observed concerning FEV1 (forced expiratory volume in 1 s). Tremor, measured with an opto-electronic tremorgraph, was about the same for two treatments and not significantly different from tremor seen in healthy children. The reported side effects were less frequent in the SR tablet period.