The Effect of Oestrogens Alone

()estrogens show a variety of effects on the mammalian corpus luteum, depending on the stage of the oestrous cycle at which they are administered. This report discusses an investigation of the effect of a single injection of either mg oestradiol benzoate or 5mg oestradiol valerate, at all stages of the cycle in the cyclic cow. The concentration of plasma progesterone in daily peripheral blood samples, the apparition of oestrus and the occurrence of ovulation and pregnancy were the criteria used to evaluate the effects on the corpus luteum. A multiplicity of effects was the result : luteotrophic, or antiluteotrophic at the beginning of the cycle, luteolytic during most of the luteal phase, and anti-luteolytic at the end of the luteal phase. The same criteria were used to evaluate the combination of treatment with a synthetic progestagen (norethandrolone or SC 2I oog) with a single injection of oestradiol valerate treatments in current use for the synchronization of oestrus in cattle with particular reference to treatments begun early in the oestrous cycle (Days 3 and 5), when an injection of this oestrogen alone has no luteolytic effect. It appears that SC 2l oog in association with oestradiol valerate has a much greater inhibitory effect on the development of the corpus luteum than does the association of norethandrolone with oestradiol valerate.