To investigate the potential importance of oestrogen as a local regulator of human corpus luteum function, the mRNA expression pattern and cellular localization of oestrogen receptors (ERs), ER-alpha and ER-beta, were studied in corpora lutea grouped according to age, where days 2-5 post-LH rise were designated as the early luteal phase, days 6-10 as mid-luteal and days 11-14 as the late luteal phase respectively. Northern blot analysis using an ER-beta probe in samples from whole ovarian tissue and isolated corpora lutea, revealed a major band at 7.5 kb and several minor bands between 4-10 kb, while no signals for ER-alpha mRNA were obtained. However, using a semi-quantitative reverse transcription-polymerase chain reaction followed by Southern blotting, ER-beta mRNA levels were found to be 63% lower (P: < 0.05, n = 39) in the mid-luteal phase compared with the early luteal phase, while ER-alpha mRNA expression showed no statistical differences between the different age groups. Using in-situ hybridization, ER-beta mRNA expression was localized to the steroidogenic luteal cells as well as perivascular cells and fibroblasts in the corpus luteum. Immunohistochemistry confirmed the localization of ER-beta protein, but no clear staining of luteal cells was found using antibodies against ER-alpha. Collectively, the findings of low to moderate expression of ER-beta mRNA and protein in the steroidogenic cells, and also in vascular endothelial cells of the corpus luteum, as opposed to diminutive amounts of ER-alpha mRNA, suggest that oestrogen activity is primarily transduced via ER-beta in the human corpus luteum.