Samples of peripheral blood were taken from 4 intact rhesus monkeys every 3 hr for a 72–hr period encompassing the midcycle LH surge, and assayed for LH, FSH, estradiol, estrone and progesterone. The LH and FSH peaks were essentially parallel. They were continuous, symmetrical, and monophasic, with a mean duration of approximately 50 hr. The rate of decrease in LH concentration during the descending phase of the surge did not approach the half-life of LH in the monkey, indicating that secretion was continuing during this time. A sharp rise in estradiol accompanied the initial increase in LH, resulting in a peak which preceded that of LH by 9–15 hr. Progesterone was undetectable in peripheral plasma until 15 hr after LH began to rise, reaching a small peak within 6 hr of the LH peak. Estradiol started to decrease before the decrease in progesterone, but both steroids reached nadirs approximately 24 hr after the LH peak. In a separate group of 12 animals, the earliest evidence of ovulation was obtained appr...