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The giant panda is critically endangered in its native habitat, which is continually depreciated by the pressure of human population growth, and the small captive population is not yet self-sustaining. Female giant pandas are seasonally monoestrous with spontaneous ovulation. Precise monitoring of the estrous cycle to pinpoint the time of ovulation is critical for the success of both timed mating and artificial insemination. Vaginal cytology is a useful adjunct to endocrinological data in describing the estrous cycle in many species, including domestic carnivores; however, it has not been used effectively in the giant panda. In this study, vaginal swabs were obtained from four nulliparous females ranging in age from 6 to 14 years. Cells were stained with Papanicolaou stain and classified as basophils, intermediate cells, or superficial cells based on traditional criteria. In addition, cell color changes from the basophilic through the acidophilic and keratinized stages were recorded. The size and color of the vulva of a single female was scored daily on a scale of 0-3, with anestrus characterized as 0. The day of ovulation was retrospectively designated as day 0 based on urinary estrogen concentrations, behavior, and morphological indicators. The vulva began to swell between days -11 and -9, with peak tumescence on the day of ovulation. The percentage of superficial cells began to increase by day -8 to -7, and peaked between days -2 and +2. A chromic shift occurred on day -8, when the majority of vaginal cells changed from basophilic to acidophilic without any accompanying nuclear or cytoplasmic changes. A second chromic shift was observed on day -2, when keratinized cells replaced acidophilic cells as the majority of vaginal cells. By day +4, all parameters had returned to baseline except vulvar swelling, which slowly subsided over the next several days. These data show the correlation of trichrome-stained vaginal cytology with other measured indicators of estrus in the giant panda, and its predictive value in determining the time of ovulation.