A total of 595 menstrual cycles were studied in 28 adult M. fascicularis over a period of five years. Ovulations, observed laparoscopically in 138 cycles, occurred on the left ovary 62.6% of the time. No significant effect was noted on cycle length relative to the ovulatory status of the previous cycle or the occurrence of ovulation on the same ovary in consecutive cycles. Laparoscopy had no effect on the parameters studied.