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Reproductive tissues (uterine endometrium, corpus luteum and ovarian residual tissues) from pregnant and pseudopregnant rabbits were incubated with equimolar concentrations of [³H]oestrone and [³H]oestrone sulphate (0·375 pmol) to monitor the changes in oestrogen metabolism during the early stages of pregnancy (days 0, and 3–8 post coitum) and to investigate the embyonic effect upon maternal oestrogen metabolism.

Oestradiol-17β was the major metabolite formed from oestrone and sulphoconjugation occurred in all tissues studied. Oestrone sulphate was converted primarily to oestradiol-17β-3-monosulphate. Endometrial 17β-oxidoreductase significantly decreased and sulpho-transferase increased in activity during the preimplantation period, but no differences were noted between gravid and non-gravid horns in unilaterally pregnant animals, nor between pregnant or pseudopregnant animals. Significant decreases occurred in 17β-oxidoreductase and sulphotransferase activity in luteal tissue, but these were more than offset by increases in tissue weight. No differences in the activities in luteal tissues were detected between pregnant or pseudopregnant animals, nor between ovarian tissue adjacent to gravid or non-gravid uterine horns.

The results show that significant changes occur in oestrogen metabolism in the rabbit endometrium and corpus luteum within 8 days after ovulation, and that these changes result from maternal factors expressed systemically rather than by the effects of the developing conceptus expressed locally.