To explore the effects of soybean isoflavone (SIF) on expression of estrogen receptor alpha (ERα) and interleukin–2 (IL–2) protein as well as mRNA in the pituitary of ovariectomized rat, forty healthy female rats were ovariectomized, and then respectively supplied with SIF at high dosage (1.5 mg/kg), middle dosage (1.0 mg/kg), low dosage (0.5 mg/kg) and solvent agent by mean of hypodermic injection, at the same time, sham operation group was injected at the same amount of solvent agent. After treatments of the second and sixth week, 5 rats out of each group were randomly selected for the study. The expression and distribution of ERα, IL–2 protein and mRNA in the pituitary were studied using technique of immunohistochemical (IHC) and in situ histo–hybridization method (ISHH). The results showed that the general tendency of intensity and the number of positive cells of ERα and IL–2 in pituitary of ovariectomized group rats were significantly decreased with time after ovariectomized. While, in ovariectomized rats treatment with SIF, the general tendency of the intensity and the number of ERα and IL–2 positive cells were increased, especially at high doses of soy isoflavones for six weeks, the intensity and quantity of these products were even reached to the level of that in the sham–operation group, while, in other groups, only part were recovered with SIF dose and time increase. The above results indicated that SIF could upregulate the expression of ERα and IL–2 protein as well as mRNA in the pituitary ovariectomized rat, and it showed a time and dose dependent manner.