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mechanisms NM WILSON, PJ BARNES, H VICKERS, M SILVERMAN Departments ofPaediatrics and Neonatal Medicine and of Medicine, Hammersmith Hospital, London ABSTRACr The mechanism by which airway cooling induces airflow obstruction in asthmatic subjects has not yet been established. Using a pair of isocapnic hyperventilation challenges, with a 40-minute interval, we looked for the presence of a refractory period in 19 asthmatic patients (aged 9-18 years). The subjects fell into two groups. The eight in the "non-refractory " group showed less than a 25 % reduction in response to the second challenge, but the 11 in the "refractory " group showed at least a 35 % reduction. Twelve subjects also performed a hyperven-tilation challenge after cholinergic blockade with inhaled ipratropium bromide. In five, in whom no refractoriness after hyperventilation was seen, there was a significant protection from cholinergic blockade (p < 0.05). In these a vagal (cholinergic) reflex seems likely. The remaining seven, who had a refractory period, received no significant protection from cholinergic blockade and therefore no evidence for the presence of any cholinergic mechanism. We conclude that two mechanisms are responsible for hyperventilation-induced asthma, one of which is a vagal refle