Of the four rhesus monkeys that conceived during the study, two were bred 12.5 h post-estrogen peak. The precise temporal relationship between the estrogen peak and breeding could not be determined for two of the rhesus monkeys; there is, however, good evidence that breeding occurred within 18-24 h post-estrogen peak. Fertile matings in four bonnet monkeys occurred at 8.0 h pre-peak and 7.5, 12.0, and 12.5 h post-peak. The optimal mating period for both macaque species appears to be limited to an approximately 24 h period at and immediately after the estrogen peak. The temporal position of the preovulatory estrogen peak in relation to the fertile period of the menstrual cycle makes it an excellent indicator for use in detecting an impending fertile period. The apparent brevity of the optimal breeding period and its occurrence 24-36 h prior to the reported time of ovulation suggest temporal relationships regarding breeding, ova and sperm transport, gamete viability, and fertilization which were previously unknown.