Circulating levels of LH [luteinizing hormone], FSH [follicle stimulating hormone], estradiol and progesterone were measured by radioimmunoassays in daily serum samples throughout the menstrual cycle of 7 adult cynomolgus monkeys [Macaca fascicularis]. The composite pattern of these hormones in cynomolgus monkeys strongly resembles that reported for rhesus monkeys, and is similar to that in the menstrual cycle of women. In the face of diminishing supplies and higher costs of rhesus monkeys, the cynomolgus monkey can also serve effectively as a model for studies in primate reproduction.