The experiment described in this paper initially aimed at developing a simple inexpensive method of pregnancy diagnosis for sheep. We considered that, if oestrus could be induced in the non-pregnant ewes in a flock (which may be in anoestrus) and this oestrus detected by the use of harnessed rams (Radford, Watson & Wood, 1960), a method of pregnancy diagnosis could be developed.Diamond & Young (1963) present evidence from a number of species that pregnancy prevents the virilizing effect of a range of exogenous steroid substances. Lindsay & Francis (1969) and Fels (unpublished data) have shown that ovariectomized ewes do not respond to oestrogen while under the influence of exogenous progesterone. Thus, endogenous progesterone could be expected to prevent oestrous responses in pregnant ewes.One
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