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In intact guinea-pigs the concentration of progesterone in luteal tissue rose and fell in a pattern similar to that of the volume of the corpus luteum, which in pregnancy grew to nearly twice the average size attained in the normal cycle. The concentration of progesterone in arterial plasma remained low throughout the normal cycle, but in pregnancy it increased at least ten-fold.After hysterectomy, when the corpora lutea persisted for a long time and were comparable in size and in progesterone content to those of pregnancy, the levels in arterial plasma were very low, and were no higher than in the normal cycle.After hypophysectomy early in the normal cycle, about one-half of the corpora lutea persisted for at least 5 weeks, were histologically normal in appearance and eventually reached a volume similar to that of corpora lutea in the normal cycle. If the operation was performed later, luteal regression was not delayed. The concentration of progesterone in the luteal tissue was again comparable to that found in pregnancy, whereas arterial levels remained extremely low, as after hysterectomy.After hypophysectomy of mated animals, implantation was not affected, and a high proportion of embryos survived if the operation had been performed 2 or 3 days after ovulation, the size and histological appearance of the corpora lutea being similar to those of intact females at corresponding times in pregnancy. The arterial concentration was only comparable to that of normal pregnancy in animals in which apparently normal conceptuses survived.After hysterectomy at different times after hypophysectomy, the corpora lutea in some animals increased to a size comparable with that in pregnancy, but in others there was little change. Plasma progesterone levels were low.In general the concentration of progesterone in luteal tissue was closely related to the size of the corpora lutea except in the instance of 537 538 R. B. Heap et al.the corpora lutea formed at the post-partum oestrus. Arterial progesterone levels, on the other hand, were high only in pregnant animals with healthy embryos, whether they were intact or hypophysectomized.

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