Abstract: AimTo study the effects of human recombinant interleukin-1 receptor antagonist (IL-1ra) on isolated trachea smooth muscle (TSM) of the guinea pig. MethodsChanges of the tension of isolated trachea were measured by force-displacement transducer and MedLab recording system in vitro. ResultsIL-1ra showed direct relaxed effect on TSM in normal and ovalbumin sensitized guinea pig. The EC₅₀ values were 8.06×10^-8 and 5.88×10^-7 mol·L^-1 respectively. IL-1ra(1×10^-7-1×10^-5 mol·L^-1) concentration-dependently inhibited the contraction of TSM induced by 1×10^-3 mol·L^-1 histamine(His), 1×10^-3 mol·L^-1 acetylcholine (ACh) and 1×10^-6mol·L^-1 5-hydroxytryptamine(5-HT) (P<0.05 or 0.01). When IL-1ra was given in advance, the contractile actions of His, ACh and 5-HT were antagonized by IL-1ra (1×10^-7-1×10^-5 mol·L^-1), the pD′₂value for His was 6.6±0.3. However, the contractile action of ACh was enhanced by IL-1ra at low concentration of 1×10^-9-1×10^-8 mol·L^-1. IL-1ra significantly prevented and inhibited the contraction of sensitized TSM induced by antigen ovalbumin，the IC₅₀ value was 4.48×10^-7 mol·L^-1. ConclusionThe results indicate that IL-1ra, within certain concentration, can relax the normal, contracted and sensitized ISM of the guinea pig in vitro.