The authors have evaluated the effectiveness of the protection of a new beta 2-adrenergic compound, reproterol, against broncho-constriction induced by physical exercise in a group of individuals of paediatric age sensitive to broncho-stimulation. This study has been carried out comparing reproterol with salbutamol, using placebo as a control, following a randomized single-blind crossover trial. The provocation test has been performed following the instructions of the Italian Society of Paediatrics. The drugs have been administered in the oral liquid form at the dose of 0.28 mg/kg and 0.1 mg/kg of reproterol and salbutamol, respectively. The two substances have shown a similar preventive effectiveness in controlling exercise-induced asthma up to 2 hours from administration with reproterol having a stronger action at the beginning of the observation.