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Summary. The interactions of oestriol and progesterone were studied in a series of assays for oestrogenic and progestational activities, and the responses were compared with data on progesterone-oestrone combina¬ tions in the same tests. The vaginal effects of these two oestrogens do not seem to differ, whereas the interactions of oestriol and progesterone are quite different from the interactions of oestrone and progesterone when uterine end-points are considered. Since oestriol is the dominant aromatic steroid excreted during pregnancy and the luteal phase of the menstrual cycle, we feel that explanations for many unsolved problems of luteal-phase and pregnancy physiology may reside in these interactions.