Aspiration of ovarian follicular contents in humans is a well-established procedure used to obtain oocytes for fertilization in vitro (IVF). However, the effects of aspiration on the menstrual cycle and resulting luteal function have been incompletely characterized. The present study was designed to investigate alterations in the temporal and endocrine characteristics of menstrual cycles following aspiration of contents of the dominant preovulatory follicle (DF) on day 10 of the cycle in normal rhesus monkeys. When aspiration was performed prior to the preovulatory surge of luteinizing hormone (LH), cycle length was extended to 38.6 ± 8.6  (x days ± SD, [n]), as compared to 29.5 ± 5.7  days when the surge occurred before the time of aspiration. Mean and maximal amounts of progesterone (P) in the luteal phase and the number of days in which P-values were > 1 ng/ml were significantly greater when aspiration was performed prior to the surge of LH than for aspiration after this event. Laparoscopic observations made in the midluteal phase in animals of the former group demonstrated that the corpus luteum (CL was derived from a follicle other than the original DF which had been aspirated on day 10 of the menstrual cycle; observations in the latter group of animals indicated that the CL was derived from the DF.