Because asthmatics who exhibit exercise-induced bronchonconstriction are also known to be sensitive to the cholinergic drug methacholine, an attempt was made in this study to block exercise-induced bronchoconstriction in nine asthmatics with a known history of exercise-induced bronchoconstriction by pretreatment with atropine. Inhalation of atropine before exercise produced milder exercise-induced bronchoconstriction in three of the nine asthmatics and a total inhibition of exercise-induced bronchoconstriction in two patients. The remaining four asthmatics still exhibited bronchoconstriction during exercise, despite pretreatment with atropine. There was no correlation between a past history of reagenic bronchial allergy or aspirin hypersensitivity and the extent of atropine inhibition of exercise-induced bronchoconstriction. This varied effect of atropine was considered to be due in part to varying amounts of the drug reaching those parts of the bronchial tres most involved in triggering exercise-induced bronchoconstriction.