Histological analysis of 29 ovaries removed at midcycle from 19 bonnet monkeys (M. radiata) revealed an unusual mode of follicular growth. All nonatretic follicles greater than 1.5 mm diameter were observed to have deeply folded walls giving the appearance of collapse due to inadequate follicular fluid production. Ovulation from these collapsed follicles apparently results from their full inflation by blood plasma after LH [luteinizing hormone] stimulation. The corpora lutea formed from this unusual follicle were not different from those observed in other macaque species except for large numbers of eosinophilic granulocytes found in the interstium. Two modes of luteal regression were observed. In 1 instance a vascular stigma resembling a fresh ovulation stigma was found over an aberrant corpus luteum. The risk of misinterpretation of such structures is explained. The morphologic observations were correlated with peripheral blood levels of total estrogens and luteinizing hormone.