The sustained elevation during Days 12\p=n-\21of pregnancy of rat luteal ornithine decarboxylase, which is inducible experimentally by hCG and PGF-2\g=a\,was dependent upon intact placentae and oestrogen, but independent of prostaglandin, and was absent at corresponding times of pseudopregnancy. It is suggested that this enzyme activity is a response to an hCG-like hormone of placental origin. Two brief reappearances of enzyme activity on Days 21 and 22 of pregnancy, dependent upon intact

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... tae and upon both oestrogen and prostaglandin, suggest a pulsatile release of PGF-2\g=a\from the uterus or placentae. These experiments illustrate the value of ornithine decarboxylase in seeking unknown hormonal influences upon the corpus luteum.