Different cell types of the ovarian follicle have been used to study steroid biosynthetic pathways. These investigations involved the use of tissue culture (Channing & Grieves, 1969) and incubations in vitro with specific buffers (Ryan & Short, 1965; Ryan, Petro & Kaiser, 1968). The activity of these cells in biological fluids has not been extensively studied. The aim of the present investigation was, therefore, to compare plasma and follicular fluid as incubating media for the biochemical study of the 3β-hydroxysteroid dehydrogenase and aromatizing activities of granulosa cells from equine ovarian follicles obtained at oestrus.
Radioactive steroids of high specific activity were purchased from the Radiochemical Centre, Amersham, Bucks., and were checked for purity before use. Reproductive tracts were obtained at slaughter from four mares judged to be in oestrus by gross examination of the uterus, cervix and corpus luteum using the criteria outlined by Channing (1969). At the