Most of our knowledge of ovarian physiology is based upon studies that have demonstrated functional oestrogen receptors in the ovaries of lower animal species. The presence of oestrogen receptors in primate granulosa cells has been questioned by some investigators. However, we have found oestrogen receptor messenger RNA in human granulosa cells by reverse transcriptase-PCR assay. Furthermore, using immortalized granulosa cell lines transfected with a plasmid containing an oestrogen response element, a functional oestrogen receptor was confirmed. These experiments strongly support the hypothesis that human granulosa cells express biologically active oestrogen receptor.