PIP: The data on blood and urine levels of follicle stimulating hormone (FSH, luteinizing hormone (LH) and estrogens during the normal menstrual cycle are summarized. The values for plasma progesterone and the urinary excretion of pregnanediol are presented. The relationship between gonadotrophin and steroid levels with reference to the events at midcycle are considered. Studies of blood are more valuable that studies of urine to interpret the sequence of events at midcycle. In the majority of the cycles reported, the midcycle peaks of blood FSH and LH coincide. The peak of estrogen in blood at midcycle precedes the LH maximum but the time relationship is variable. During the LH peak a small rise in plasma progesterone occurs, but the major increase in this hormone does not take place until after LH values have decreased. Ovulation probably does not take place till 12-24 hours after the LH peak.