Horses with asymptomatic or symptomatic chronic obstructive pulmonary disease (COPD) and control horses were challenged with oil seed rape either by exposure to a field of flowering Brassica campestris for six days, or by an inhalation challenge with freshly collected pollen from Brassica napus, or by inhalation challenge with a commercial extract of B napus pollen. Clinical and bronchoscopic examinations showed that the challenges did not induce detectable pulmonary disease in the control or asymptomatic COPD affected horses and did not significantly affect their pulmonary mechanics, arterial blood gas tensions, arterial pH or the cytology of bronchoalveolar lavage fluid. However, the challenges with fresh B napus pollen and the commercial extract of B napus pollen exacerbated the pulmonary disease in some of the symptomatic horses, possibly owing to non-specific toxicity or non-specific bronchial hyperresponsiveness. Intradermal testing with the commercial extract of B napus pollen suggested that none of the horses were hypersensitive to this agent.