The role of estrogens in hen reproduction is well established. However, the distribution of estrogen receptors in the chicken ovary is unknown. Therefore, the mRNA expression of alpha (ERalpha) and beta (ERbeta) estrogen receptors was examined within the ovaries of laying hens. Expression of ERs was determined by RT-PCR analysis. The presence of ERalpha and ERbeta mRNAs was found in the ovarian stroma and white, yellowish, small yellow and the largest preovulatory (F3-F1) follicles. ERalpha and ERbeta mRNAs were detected in the granulosa and theca layers of the walls of preovulatory follicles. The expression of ERalpha mRNA was markedly higher than ER? mRNA in all examined ovarian compartments. Within the ovary, the relative expression of both ER mRNAs depends on the follicular diameter and the layer of the follicular wall. The results demonstrate the expression of both ERalpha mRNA and ERbeta mRNA in all compartments of the chicken ovary, suggesting different pathways of estrogen action in the avian ovary. Much higher expression of ERalpha mRNA indicates that this form of estrogen receptor is predominant in the chicken ovary. The clarification of the mechanism of ER? and ER? participation in the ovarian functions of birds necessitates further experiments examining ERs at the protein level.