Three pharmacokinetic studies of terbutaline slow-release (SR) tablets, 5 and 7.5 mg, were performed in healthy subjects. Four men and 4 women volunteered for each study. Bricanyl plain tablets were used as reference formulation. Both single- and multiple-dose treatments were performed. For repeated administration, SR tablets were given every 12 h. Plain tablets were given every 8 h in two studies and every 12 h in one. Steady-state bioavailability correlated with the amount dissolved in vitro (paddle method) in 6 h. Single-dose bioavailability was limited by the amount dissolved in 4 h. The difference may have a pharmacological explanation, in that repeated administration of a sympathomimetic drug is known to decrease gastro-intestinal motility. Tablets of both strengths with an intermediate dissolution rate were extensively tested. Plasma concentrations and the rates of urinary excretion of unchanged terbutaline were measured. Mean relative bioavailability compared with plain tablets was 76-77% with 7.5 mg SR tablets and 74-80% with those of 5 mg. Variation in bioavailability between and within subjects was the same or smaller with the SR tablets. They gave smoother plasma concentration profiles with delayed peaks and the same peak/trough concentration ratios as the plain tablets, despite less frequent dosing. However, objectively measured side-effects were not significantly reduced. Measurements after cessation of treatment gave terminal half-lives in plasma of 11.5-23.0 h which is considerably longer than reported in the literature. Renal clearance averaged 140 mL/min, similar to predicted creatinine clearance. From these studies it is concluded that the main advantage with the SR tablets is the twice-daily dosage regimen. This should increase compliance, facilitate combination therapy with prolonged-action formulations of other drugs and better maintain therapeutic levels during the whole night interval.