the release of LH. It now becomes easy to visualise how an inade-
quate progestagen regime - whether due to dose or type of steroid -
could result in an inadequate endocrine status at oestrus. Such a
regime could result in an incomplete block of FSH release so that
"leakage'" occurs, with the consequent production of oestrogen and

of LH. On withdrawal of progestagen, there may be insufficient FSH
available to induce the production of sufficient oestrogen for a
full behavioural and physiological oestrous response and LH may be
inadequate to initiate the development of a fully functional corpus
luteum.

It seems therefore that a completely effective inhibition of
release of pituitary gonadotrophin during treatment, followed by a
complete release on cessation, may be absolute prerequisites for
successful synchronized breeding. Complete release on cessation of
treatment requires the sudden withdrawal of the progestagen which
must have a duration of activity comparable to that of the natural
hormone.

Hansel (1967), in reviewing the feeding of oral progestagens to
cattle, concluded that variation in dosage contributed to two types
of failure, namely ovulation without oestrus during treatment
("breakthroughs") and ovulation without oestrus after treatment
(Msilent heats"). The former were associated with too little and
the latter with too much progestagen. We had felt that the minimum
dose of progestagen required to inhibit ovulation would give maximum
fertility. Lamond (1964, 1965) postulated an optimum dose,
depending on factors such as season of the year. Clearly the
ruminants produce very large quantities of progesterone. Thus Heap
& Linzell (1966) estimate that the ovaries of the goat produce 9 to
10.5 mg per day. Hence many of the dosages used in the past must
have been too low to duplicate all the activities of the corpus
luteum. Throughout our work on intravaginal sponges with Cronolone
there has been a persistent but elusive effect of dose, in favour of
higher dosages. Similarly Hulet (1966) reports a slightly higher
percentage of ewes lambing following oral treatment with 60 mg per
day of MAP as compared with 50 mg.

Qualitative differences between progestagens have been shown by
Hansel, Donaldson, Wagner & Brunner (1966), Lindsay, Moore, Robinson,
Salamon & Shelton (1967), Robinson et al (1967), and Shelton et al
(1967). High incidence of oestrus and high fertility are associ-
ated with short duration of activity.

The possible role of steroids of the corpus luteum, other than
progesterone, is now being considered. This has stemmed from the
observation that the pattern of development of follicles in the cow
following progestagen treatment differs from normal. Zimbelman &
Smith (1966) studied the potency of MGA in cattle and noted that (a)
a fern pattern, indicative of oestrogenic activity, developed in

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