SUMMARY: A comparison has been made between the steroids present in equine follicular fluid and luteal tissue. Follicular fluid was found to contain progesterone, 17α-hydroxyprogesterone, androstenedione, epitestosterone, 19-norandrostenedione, oestrone, oestradiol-17β and 6α-hydroxyoestradiol-17β, in confirmation of previous studies. On the other hand, luteal tissue contained large amounts of progesterone and 20α-hydroxypregn-4-en-3-one, and a small quantity of 17α-hydroxyprogesterone. No C18 or C19 steroids could be detected. These findings are therefore interpreted as evidence in favour of a 'two-cell type' theory of steroid formation in the ovary. One type, possibly the theca interna cell, has the ability to convert progesterone into oestrogens; the other type, possibly the luteinized granulosa cell, has a diminished 17-hydroxylating ability and is unable to cleave the C21 side chain to make C18 and C19 steroids. However, it does have an active 20-reductase enzyme system present.