Daily measurement of serum luteinizing hormone [LH], estradiol-17.beta. and progesterone [P] were made during the menstrual cycle in 9 pigtail macaques (M. nemestrina). All data were normalized to the day of the LH peak. Serum estradiol-17.beta. increased from approximately 100 pg/ml during the early follicular phase to 442 .+-. 156 pg/ml during the maximum midcycle concomitant with the LH peak and a small increase in serum estradiol-17.beta. was observed during the luteal phase coincident with the P peak. Serum P values increased slightly at the time of the LH peak and increased from 0.2-0.3 ng/ml during the midfollicular phase to peak levels of 8.3 .+-. 1.75 ng/ml 9 days after the LH surge. Serum LH remained low and relatively constant throughout the early and midcycle, then sharply increased approximately 4-fold to peak values of 6.25 .+-. 0.9 ng/ml. Sex skin swelling increased slowly during the follicular phase and declined slowly throughout the early luteal phase. Rectal temperature did not change significantly throughout the menstrual cycle. The similarity of plasma sex hormone changes during the menstrual cycle between women and the pigtail macaque suggested that this nonhuman primate should be a useful animal model for studying human reproduction.