Between Days 9 and 15 after oestrus, concentrations of pregnenolone, pregnenolone sulphate, dehydroepiandrosterone (DHEA), DHEA sulphate, androstenedione, oestrone and oestrone sulphate in free uterine fluid collected from non-pregnant gilts were greater than respective values in plasma (P < 0\m=.\05). The total contents of pregnenolone, progesterone, DHEA, testosterone, oestrone and oestradiol in washings from pregnant uteri exceeded (P < 0\m=.\05)respective non-pregnancy levels during this

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... e period. Concentrations of pregnenolone, pregnenolone sulphate, DHEA, DHEA sulphate, androstenedione, oestrone, oestrone sulphate and oestradiol in free uterine fluid recovered from gravid uteri were also higher (P < 0\m=.\05) than respective plasma values. By contrast, the progesterone concentration in uterine fluid from pregnant animals was lower (P < 0\m=.\001) than the plasma value. Concentrations of DHEA, DHEA sulphate, androstenedione and oestrone sulphate in plasma of pregnant gilts between Days 9 and 15 after mating exceeded (P < 0\m=.\05) the respective concentrations in unmated gilts between Days 9 and 15 after oestrus. Plasma levels of pregnenolone sulphate were lower (P < 0\m=.\05) in the pregnant animals. We therefore suggest that the endometrium of the pig can concentrate steroid hormones in uterine fluid and that increases in steroid levels in this milieu between Days 9 and 15 after coitus reflect steroidogenesis by embryonic tissues and modification of enzyme activities within uterine tissues under the influence of progestagens. The pool of steroid sulphoconjugates present in uterine fluid between Days 9 and 15 post coitum could serve as an important precursor source for progestagen, androgen and oestrogen synthesis by tissues of pig embryos before implantation.