The metabolism of [4-14C]progesterone and [4-14C]androst-4-ene-3,17-dione (androstenedione) by a granulosa-theca cell tumour of the ovary, 310 g. in weight, removed from a 43-year-old post-menopausal woman has been studied in vitro.The experiments provided no evidence that the tumour could utilize progesterone as an oestrogen precursor. However, the tumour tissue converted androstenedione to oestrone and oestradiol-17\g=b\. The major metabolite of progesterone was 20\g=a\-hydroxypregn-4-en-3-one.These results are discussed in relation to current theories of the synthesis of steroids in the ovary.