The effects of an oral preparation containing a mixture of extracts from yellow gentian, garden sorrel, cowslip, verbena and common elder on the lung function of nine horses suffering from heaves were determined in a longitudinal crossover study. The horses were divided at random into a group of five (group 1) and a group of four (group 2). The horses in group 1 were each given 15 tablets of the preparation twice daily, while the horses in group 2 were left untreated. Fourteen days later, the horses in group 2 were given the same course of treatment while the horses in group 1 were left untreated. On being subjected to a histamine inhalation provocation test, five of eight horses tested appeared to be hyperresponsive to histamine. The treatment decreased the histamine sensitivity of three of them; it also caused a significant decrease in maximal intrapleural pressure difference of all the horses. The treatment had no significant effects on the clinical signs, the mucociliary activity or the cytology of the bronchoalveolar lavage fluid of the horses.