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Utilizing paper and gas\p=m-\liquid chromatography, the pattern of steroid secretion by cultures of equine granulosa, thecal and stromal cells has been studied.Granulosa cell cultures secreted primarily progesterone and 20\g=a\-hydroxypregn-4-en-3-one. A granulosa cell culture of 700,000 cells harvested from two large vascular follicles of a mare in oestrus secreted the following amounts of steroid from days 0 to 6 of culture: 561 \g=m\g.progesterone, 68 \g=m\g.20\g=a\-hydroxypregn-4-en-3-one, 5\m=.\3 \ g = m \ g . 17-hydroxyprogesterone, 1 \ g=m\ g. androstenedione, 0\m=.\12 \ g=m\ g. dehydroepiandrosterone and 2\m=.\7 \g=m\g. oestradiol.Thecal tissue harvested from the same follicles secreted 29, 3\m=.\0,1\m=.\8,1\ m=. \ 4, 1\m=.\1and 0\m=.\99 \ g=m\ g. , respectively, of the same steroids. All these steroids originated from endogenous precursors or from acetate or cholesterol in the tissue culture medium which contained 15% horse serum, 30% medium '199' and 55% Hanks's solution.Cultures of granulosa cells harvested from two large vascular follicles of a mare in early oestrus secreted progesterone at a constant rate of 35 pg./cell/ day between days 6 and 34 of culture, thus demonstrating that these cells do not dedifferentiate in culture. The secretion of 20\g=a\-hydroxypregn-4-en\x=req-\ 3-one relative to progesterone increased from days 3 to 6 and remained constant from days 6 to 15.

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Progesterone and 20\g=a\-dihydroprogesterone were measured in the ovarian venous blood of rats in early pro-oestrus, late dioestrus, day 5 of pseudopregnancy, and in androgen-treated animals with persistent oestrus. Little progesterone was secreted in early pro-oestrus or persistent oestrus (0\m=.\1\ g=m\ g. / hr.), but the secretion rose in dioestrus (2\ m=. \ 1\g=m\g./hr.) and in pseudopregnancy (7 \g=m\g./hr.). The 20\g=a\-dihydroprogesterone secretion was low in the persistent oestrous group; in all the other groups it was 1\m=.\5\p=n-\3 \g=m\g./hr. Treatment with luteinizing hormone increased progesterone secretion within 30 min. in pro-oestrus and persistent oestrus, had no significant effect in dioestrus, and depressed it markedly in pseudopregnancy. It appeared to have no significant effect on 20\g=a\-dihydroprogesterone secretion except during pseudopregnancy, when it was depressed.Treatment with prolactin produced no effect within 30 min.in pro-oestrous, dioestrous or pseudopregnant animals.