the pituitary content has been released (Robertson & Rahka,
1966) .

(c) This release is associated with a rise in the output of oestro-
gen (mainly oestradiol 17B) which reaches a peak at or shortly
before the onset of oestrus (Moore & Brown, 1968). The pro-
duction of oestrogen falls rapidly with the cessation of
release of FSH.

(d) The release of LH commences at the onset of oestrus when oes-
trogen production is at a peak. The release is rapid - 6
hours - and concludes at the same time as does the release of
FSH (Robertson & Rahka, 1965, 1966). At this time oestrogen
production also is greatly reduced.

(e) By 12 hours after the onset of oestrus, and 12 hours before
ovulation, the release of FSH, LH, oestrogen and progesterone
has virtually ceased. Oestrogen concentration in reactive
tissues remains high (Glascock & Hoekstra, 1959) but there is
no evidence for such retention of progesterone (Ogilvie,
Casida, First & Hoekstra, 1965).

(f) Hence at the time of ovulation and fertilization the endocrin-
ological status is virtually neutral. The blood levels of all
circulating hormones are minimal. Hence it is logical to
assume that the effect of even minute deviations in the amounts
of oestrogen or progesterone would be profound. It is with
such minute deviations that we are concerned.

In the absence of any concrete evidence of the direct involve-
ment of hormones other than oestrogen and progesterone, let us
assume that only these two are involved in the phenomena associated
with sperm transport and fertilization and survival of the ovum.
What do we know of the factors which control their production and
how are these affected by treatment with exogenous hormones?

First, progestagen administered to a ewe after the formation of
the corpus luteum is well initiated (day 4) does not affect its
production of progesterone nor its functional life (Smith & Robinson,
1968). If given while the corpus luteum is being formed (before
day 4) its life is reduced (Petrov, 1967; Woody, Ginther & Pope,
1966; Woody, First & Pope, 1967) and its production of progesterone
ceases by the 12th day (Smith & Robinson, 1968). Data for the cow
are less conclusive, but Loy, Zimbelman & Casida (1960) and Woody
et al (1967) have presented data which suggest a similar phenomenon.
Hence towards the end of treatment, all animals in a flock or herd
are completely dependent on the administered progestagen. Hence
this must duplicate exactly the effects of an active corpus luteum.
On withdrawal of treatment, the decline of progestagen activity must
be identical with that which follows normal regression of the corpus
luteum.

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