In a group of patients with mild asthma the inhalation of mist derived from ultrasonically nebulised distilled water caused an increase in cough and a fall in FEV1. Double blind administration for five minutes of sodium cromoglycate (from an original solution containing 30 mg/ml) or atropine (2 mg/ml) by inhalation from a Minineb nebuliser, 30 minutes before the mist challenge, caused a significant reduction in the fall in FEV1 (p less than 0.05), but not in cough, by comparison with the protection afforded by placebo (saline). In a second study the fall in FEV1 caused by the inhalation of distilled water was not significantly different from that seen in response to hypotonic sodium chloride (1.7 g/l, 58 mmol/l), but both produced a significantly greater fall than did a similar mist containing sodium cromoglycate at an original concentration of 10 mg/ml (58 mmol/l). The results show that both atropine and sodium cromoglycate can block the fall in FEV1 due to mist and that protection by sodium cromoglycate is immediate. These results suggest that sodium cromoglycate blocks the nervous reflexes concerned in the response to mist, probably in the afferent limb of the reflex.