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Sheep is a species that shows 85% overall identity of the adenosine A 3 receptor with its human ortholog. In this study we demonstrate a role for the adenosine A3 receptor in ovine allergic airway responses. In isolated tracheal preparations taken from sheep naturally sensitive to Ascaris suum, antigen challenge evoked airway smooth muscle contraction. The smooth muscle contractile response was dependent on the applied Ascaris suum antigen dose. The response to a threshold dose of antigen was enhanced in the presence of an adenosine A3 agonist AB-MECA (10-6M), suggesting that adenosine A3 receptor function contributed to the enhanced allergen response. This was confirmed by showing that a selective adenosine A 3 receptor antagonist SSR16142, inhibited the increased antigen response with EC 50 =2•10 -9 M. We then tested the effects of SSR161421, given intravenously, on antigen-induced responses in allergic sheep following inhalation of Ascaris suum, in-vivo. SSR161421 (0.3-3 mg/kg) provided dose-dependent inhibition of the antigen-induced late bronchoconstrictor responses and the post antigen-induced airway hyper responsiveness. This is the first demonstration that a selective adenosine A 3 receptor antagonist inhibits antigen-induced responses in a species with an A3 receptor that bears significant similarity to its human counterpart.