Nocturnal asthma is a common and potentially dangerous condition. The mechanisms in its production are multifactorial. One factor appears to be increased vagal tone at night so that an anticholinergic drug might prove effective. The evidence for a bronchodilator action of anticholinergic drugs is reviewed. A clinical trial of one such anticholinergic in nocturnal asthma is described. In 12 patients with morning dipping a dose of 160 micrograms of ipratropium bromide inhaled at night significantly reduced the extent of morning dipping. The implications are discussed.