Purpose: To examine whether the bronchodilatory effect of atropine differs in the evening from the morning. Methods: Thirteen adult healthy volunteers with no oral medication intake were studied. At 1600, peak expiratory flow (PEF) was measured three times, and the highest value recorded. Subsequently, the volunteer received 0.01 mg·kg -1 atropine im, and the PEF was measured every 30 min for 180 min. On a different day, at 0400, the effect of atropine on the PEF was measured again in the same way. Results:The PEF values before atropine at 1600 and 0400 were 485 ± 92 (350-730) and 458 ± 76 (340-600) l min -1 ,(P < 0.05). There was no difference in PEF values between the 1600 and 0400 time courses after atropine. The PEF value was increased only at 90 min at 1600 (P = 0.0012), but at 30, 60, 90 and 120 min at 0400 (P = 0.0001).Conclusion: Atropine administration has a weak bronchodilatory effect in the evening, but a stronger effect in the morning. Airways are narrower in the morning than in the evening, and this change is inhibited by atropine such that the PEF values are restored to those observed in the evening. Conclusion : L'administration d'atropine a un faible effet broncho-dilatateur en soirée, mais un effet plus important le matin. Les voies aériennes sont plus étroites le matin que le soir. Ce qui est inhibé par l'atropine. On retrouve ainsi, le matin, les valeurs du DEP observées en soirée.