1. Lung function was studied in fifty-six symptom-free asthmatic subjects after the inhalation of urea, an agent not previously used for asthma provocation. The effects of urea were compared with those produced by the inhalation of methacholine and of isoprenaline.

2. After urea ventilatory capacity was impaired in 68% of subjects and improved in 16%. This response was a relatively constant phenomenon in the same subject. Urea appears to possess the unusual property of being able to produce both bronchoconstriction and bronchodilatation. The response could not be related to any of the clinical or other physiological variables examined.

3. Mild hypoxaemia, ventilation-blood-flow inequality and impairment of gas transfer also occurred after urea administration.

4. Methacholine produced considerable impairment of ventilatory capacity with consistent hyperinflation but with variable changes in ventilatory pattern and distribution of ventilation.

5. Isoprenaline rapidly and completely reversed the changes which occurred after urea and methacholine administration.

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