progesterone requirements. In rats, there have been suggestions that oestradiol has a sparing action on the progesterone 1 Physiology, Biomedical Sciences Division, King's College, Strand, requirement during mid-pregnancy (e.g. Seda et al., 1967) , but London WC2R 2LS, and 2 Department of Veterinary Preclinical no evidence for this was found in mice (at least after day 14: Sciences, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK Hall, 1957). The experimental study of the minimal

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... one requirements for pregnancy requires delivering physiological 3 To whom correspondence should be addressed minimal progesterone support required for pregnancy in mice varies considerably at different stages of pregnancy and is at least partly modulated by oestradiol. Materials and methods