Objective To study the relationship between Th1/Th2 cytokines messenger ribonucleic acid (mRNA) expression and immune tolerance to cardiac allografts in rats. Methods Male DA rat hearts were transplanted to male Lewis rats using Ono’s model and randomly divided into three groups: control group, rejection group, and tolerance group (each group 10 rats). Mean survival time (MST), histological changes, mRNA expression level of Th1/Th2 cytokines interleukin-2 (IL-2), interferon-γ (IFN-γ), interleukin-4(IL-4), interleukin-10(IL-10) were measured. Results MST (85.28±7.48 d) of heart allografts in tolerance group was significantly longer than that(7.33±1.03 d) in rejection group. Only a few inflammatory cells infiltrated in cardiac allografts in tolerance group. The mRNA expression of IL-2, IFN-γ (Th1 cytokines) in rejection group were much ber than those in control group, and in tolerance group were much lower; mRNA expression of IL-4, IL-10 (Th2 cytokines) in rejection group were much ...更多lower than those in control group,and in tolerance group were much ber than those in control group. Conclusions The dynamic equilibrium of Th1/Th2 cytokines is very important in immune tolerance. The deviation of Th1 to Th2 is one of the mechanisms in immune tolerance.