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On four occasions, four horses with heaves and four horses with small airway inflammatory diseases inhaled 0.9% saline based aerosol mixtures with or without lipopolysaccharides (LPS). Prior to the first saline and LPS inhalation, horses were untreated, while three and a half days prior to the third and forth inhalation horses had received 0.8 μg/kg clenbuterol intravenously twice daily.The messenger RNA (mRNA) expression of tumour necrosis factor-α (TNF-α), interleukin (IL)-1β, IL-4, IL-6, IL-8, IL-10 and interferon- γ (IFN- γ) was investigated by RT-PCR, all of which were expressed in the white blood cells of samples collected.Inhalation of LPS only changed the cytokine expression profile of IL-10, IL-4 and TNF-α mRNA which were higher after challenge with LPS. However in those horses that were treated with clenbuterol the LPS-induced IL-10 mRNA expression was shown to be suppressed. Further changes in IL-4 and TNF-αwere not significant. Thus the results of this study indicated that clenbuterol can modulate the expression of IL-10 mRNA in peripheral white blood cells in those horses with small airway diseases that have been exposed to LPS.