A colony of Macaca fascicularis have been under continuous observation for the past 4 years to ascertain the follicular morphological changes that occur prior to ovulation and during the development of the corpus luteum. Of 609 experimental cycles, laparoscopy was performed at least once in 44-5% of the cycles. Of the 104 cycles where the ovulatory status was definitely known, 89-4% were deemed ovulatory and 10-6% anovulatory. The presence or absence of ovulation in the previous cycle did not have an effect on the cycle length either for the total cycles or when analysing only cycles over 28 days. Similarly, the occurrence of two consecutive ovulations (in consecutive cycles) on the same vs. opposite ovaries did not have a significant effect on the cycle length. Neither laparoscopic stress or anaesthesia effected the normal cyclicity of the animals. The characteristic changes in follicular morphology are most clearly defined in M. fascicularis. In this species the 24 of 36 hr prior to ovulation are accompanied by discrete changes which occur in a fixed sequence, allowing one to predict the time of ovulation with reasonable accuracy. In S. sciureus ovulation is preceded by extensive bulging at the follicular apex and haemorrhaging at the base of the follicle. Due to this haemorrhaging post-ovulatory follicles in S. sciureus are generally more easily discernible than in fascicularis. The formation of clear areas (stigma) is not as evident in either S. sciureus or G. senegalensis as in the macaque. Actual ovulation has been observed four times in M. fascicularis and twice in S. sciureus.