The adult female bonnet monkey has been studied by: (a) cervical mucus weight, spinnbarkeit and ferning, (b) vaginal cytology, (c) sex skin changes, (d) uterine and ovarian size and consistency, and (e) urinary estrogen excretion. Daily measurements were made during 4 cycles, the ovulatory character of which was validated laparoscopically. Cervical mucus weight proved to be a valuable index which correlated closely with endogenous estrogen levels. Aspiration of the mucus also enabled vaginal cytology to be studied with the aid of an irrigation technique. The ovulatory cycle of the bonnet monkey is shown to be of the 4-week type, with a characteristic pattern of changes in these indices.