The corpus luteum, in its function as an endocrine gland, has a definite inhibitory effect on ovulation and menstruation or estrus. The suppression of these phenomena during pregnancy is an example of this effect. The removal of the corpora lutea abolishes this inhibitory effect and accelerates the appearance of the next period, as was first demonstrated experimentally by Leo Loeb.1In experiments performed along similar lines with guinea-pigs, I2found that after complete removal of the corpora lutea the ovulation and estrus return on the eleventh day, while in normal guinea-pigs estrus usually returns at the fifteenth or the sixteenth day. These findings indicate that the corpora lutea exert an inhibitory action, suppressing and delaying the process of ovulation in the rhythmic function of the ovary. I have used this inhibitory function of the corpus luteum as a criterion for studying luteal extracts and especially for the recognition