To establish relative protection against methacholine and histamine, 40 μg of ipratropium bromide, an anticholinergic compound, 1.3 mg of metaproterenol or placebo aerosols were administered by metered-dose inhaler prior lo inhalation challenge with methacholine or histamine in nine asthmatic subjects. Double-blind, randomized challenges were performed. Subjects required a mean methacholine dose of l.72 ± 0.73 and 2.46 ± 0.72 (Ln inhalation units), and mean histamine dose of 2.l6 ± 0.65 and 2.68 ± 0.49, to cause a drop of 20% and 35% respectively in the FEV1 following the placebo. In the methacholine challenges, both ipratropium bromide and metaproterenol had significant protection as compared to placebo (P<0.001). There was no statistical difference in the degree of protection against methacholine between ipratropium bromide and metaproterenol. In histamine challenges, metaproterenol had significant protection as compared to the placebo, while ipratropium bromide did not protect against histamine.