Chronic beta2-adrenergic stimulation with beta2-agonists has been shown to affect contractile function of skeletal muscle and to induce muscle hypertrophy in animals that underwent endurance training. In humans, on the other hand, no studies have investigated effects of beta2-agonists and endurance training on contractile function and muscle hypertrophy. Furthermore, the doses of beta2-agonists administered to animals exceed that clinically relevant for humans. Thus, the purpose of the present study was to investigate the effect of four weeks of therapeutic inhalation of the beta2-agonist terbutaline on maximal voluntary contraction (MVC) of the quadriceps muscle and leg lean mass following endurance training. Twenty-one healthy men were randomised into either a terbutaline-group (TER, n=12) or a placebo-group (PLA, n=9). Subjects underwent two visits before and after a four-week intervention. At the two visits before and after intervention, subjects’ leg lean mass was determined by dual X-ray absorbance and contractile function of the quadriceps muscle was measured during MVC with transcutaneous electrical muscle stimulation. The four-week intervention consisted of supervised endurance training on a training bike three times a week that consisted of 30 min cycling at 80-85% of maximal heart rate interspersed with 30-s of maximal sprinting every 10 min. During the four-week intervention, subjects inhaled either terbutaline (8×0.5 mg) or placebo once daily according to their respective group. Leg lean mass did not change with the intervention in either group, being 25±1 and 25±1 kg before and after the intervention in TER, and 26±1 and 26±1 kg in PLA, respectively (mean±SE). MVC, time-to-peak twitch force and half-relaxation time did not change with intervention in either group (table 1). A significant interaction of group x time (P≤0.05) was observed for peak twitch force, in which peak twitch force increased by 73 N with the intervention in TER compared to PLA. However, no within-group changes were observed in peak twitch force with the intervention in TER (309 ± 27 vs. 343 ± 27 N) and PLA (336 ± 33 vs. 297 ± 33 N). In conclusion, the present findings indicate that contractile function of the quadriceps muscle and leg lean mass are unaffected by chronic therapeutic inhalation of beta2-agonists in response to four weeks of endurance training in healthy men.