SUMMARY: The following steroids have been identified in the follicular fluid of the cow (results expressed as μg./l.): pregnenolone, 3·3 μg.; progesterone, 233·0 μg.; 20β-hydroxy-pregn-4-en-3-one, 23·0 μg.; 17α-hydroxyprogesterone, 37·6 μg.; androstenedione, 31·5 μg.; testosterone, 22·1 μg.; oestrone, 5·4 μg.; oestradiol-17β, 94·0 μg. It was not possible to detect any cortisol, 19-norandrostenedione, epitestosterone, dehydroepiandrosterone or 6α-hydroxyoestradiol-17β, although each of these steroids has been identified in equine follicular fluid. It therefore seems as if there are qualitative as well as quantitative differences between species with respect to the steroids present in follicular fluid. However, progesterone, 17α-hydroxyprogesterone, androstenedione, oestrone and oestradiol-17β are present in bovine, equine and human follicular fluid, and oestradiol-17β is the predominant oestrogen in all three species.