The twofold purpose of this study was 1) do determine whether it was possible to define the period of evolution in rhesus monkeys in terms of vaginal exfoliative changes, 2) to determine how reliably these changes continued to be associated with ovulation in an animal selected for extended study. 40 laparotomies were performed on a colony of monkeys between the fall, 1956, and spring, 1958, to confirm ovulatory menstrual cycles. Vaginal smears were taken from the area of Muellerian duct derivate (ectocervix). The smears were stained with the Shorr differential stain after rapid fixation in alcohol. The degree of cornification was assessed in percentile of cornified cells in the smear. Lavage sedimentations were concurrently recorded. In the 14 animals studied, 12 yielded daily smears intermenstrually that systematically contained a greater percentage of cornified cells than any of the daily smears of anovulatory cycles during the corresponding period. By castrating one monkey during a menstrual cycle at a critical time, it was possible to determinate the vaginal smear condition that occurs simultaneously with the onset of the period of corpus luteum formation. Replacement therapy given to two castrated animals made it possible to demonstrate that the vaginal mucosa proliferates under estrogen administration, and that the vaginal smear is a more specific index of this developmental process than is sedimentation in the vaginal lavage. An examination of cervical mucus for glucose content in nine animals was made, glucose in amount sufficient to alter glucose-sensitive test paper could not be confirmed, and this test as an index of ovulation has proven to be of no diagnostic value.