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Certain structural and functional aspects of the regression of the corpus luteum in the sheep were studied using biochemical, histological, electron microscopic and histochemical techniques. Alterations in the size and density of mitochondria in the lutein cells on Day 12 or 13 of the oestrous cycle were the first sign of luteal regression seen. This was followed by the appearance of cytoplasmic lipid droplets on Day 13 or 14, in the large, definitive lutein cells. The rapid decline in the secretory activity of the corpus luteum on Day 15 was associated with a reduction in Δ5-3β-hydroxysteroid dehydrogenase and diaphorase activities, as well as with shrinkage of the lutein cells and pyknosis of their nuclei. Despite a general correlation between all the above regressive changes, however, there was in a few instances a lack of complete concordance between the parameters studied. It is therefore not possible to state that any one criterion gives a consistently reliable indication of the functional state of the corpus luteum. A good correlation was found between the progesterone concentration in the ovarian venous blood and that in the luteal tissue during the regression of the corpus luteum. The occurrence of some regressive changes on Day 12 may be related to the fact that this is the time when the ovine corpus luteum must be stimulated by an embryo in the uterus to survive.

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