The morphofunctional state of sheep reproductive system of sheep is determined by sex hormones, which act through specific receptors triggering a series of cellular, metabolic and proliferative dependent expression of numerous genes. Unlike other mammals, in sheep endometrium and possibly other reproductive organs the oestrogen receptors are physiologically active from the prepubertal stage, with a function not yet understood. Information on the expression of sex hormone receptors in the reproductive system is very poor, particularly in the ovary and there are no studies that correlate breed with the expression of these receptors. Generally, breeders privilege higher level of prolificacy breed and this could be related to the level of oestrogen receptors expression in the reproductive system. The aim of this study was to evaluate the transcriptional expression of the oestrogen receptor in the ovary of Texel high prolificacy and Criolla Araucana standard prolificacy prepubertal sheep bred by RT-PCR quantitative real time analysis.