The efficacy of pretreatment with two doses of nedocromil sodium (4 and 8 mg) and sodium cromoglycate 12 mg were compared with placebo in inhibiting the bronchoconstriction induced by inhalation of ultrasonically nebulized distilled water ('fog') in 10 subjects with intrinsic asthma. Each fog challenge consisted of three inhalations of 30, 60 and 120 s duration, respectively, given at 4-min intervals, and the bronchoconstrictor response was assessed as the postchallenge percentage fall in FEV1 from baseline. Statistically significant drug effects (p less than 0.05) were observed after 120 s of fog challenge: both nedocromil sodium 4 and 8 mg and sodium cromoglycate were significantly more effective than placebo in inhibiting fog-induced bronchoconstriction; in addition, nedocromil sodium 4 mg proved statistically significantly more effective than sodium cromoglycate.