The activities of six oestrogenic substances were compared when given as a single injection, intravenously (i/v) or intramuscularly (i/m), to spayed progesterone-primed ewes. The following values for the ED50 for oestrous behaviour, with associated 95% fiducial limits, were obtained:
They fell into three groups, with the following i/v : i/m ratios: natural oestrogens; oestradiol- 17β, oestradiol benzoate, 4 : 1: synthetic oestrogens; diethylstilboestrol and hexoestrol, 1 : 1 : pro-oestrogen; methylethylstilboestrol, ∞ : 1.
True oestrogens are presumed to accumulate in the reacting tissues with variable efficiencies. Pro-oestrogens, given i/v, are probably ineffective because they are not converted to oestrogens in a manner or at a rate suitable for such accumulation.
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