Publisher Summary This chapter focuses on the interactions between estrogens and progesterone. The two female sex hormones—the estrogenic and the progestative substances—do not act independently. They are bound by functional relations, which constitute one of the many aspects of the vast problem of endocrine balance. It has been shown that the joint action of the two female hormones can determine mating. The females of different species do not behave in a uniform manner. In all species, estrogen produces heat to a greater or lesser extent, but in some, namely the goat and sheep, progesterone opposes this action. In others, on the contrary, progesterone reinforces the action of estrogen; this synergistic action is found in the guinea pig, in the rat, in the mouse, and in the golden hamster. In this chapter precise examples have been given of hormonal synergism between estrogen and progesterone. Hypotheses have been formulated to explain the mechanism of this synergism. It has also been suggested that estradiol can assist the action of progesterone because it increases the circulation of blood in the uterus.