Serial clinical, colpo-cytological and endocrinological examinations of two five-year-old females of the crab-eating dog (Cerdocyon thous), from the RIOZOO Foundation in the State of Rio de Janeiro - Brazil, were carried out over a 10-month period. Clinically healthy animals were kept in sand substratum enclosures, located 500m apart from each other. They were each housed with two males. The colpo-cytological technique employed for Cerdocyon thous used methods similar to those developed for domestic bitches. Unlike domestic dogs, blood cells were absent in all phases of the estrus cycle, including the pro-estrus phase. Differentiation of each type of vaginal cells during the estrus cycle phases in this species follows the same patterns shown by domestic bitches. The estradiol and progesterone levels were similar to those occurring in domestic bitches. The progesterone levels reach their maximum (46 ng/ml) around the 10th day of pregnancy. The estradiol analysis demonstrated that, although levels of this hormone could be high at various times throughout the year, mating actually occurs in late winter and in spring. It was impossible to evaluate whether males and females kept in close proximity throughout the entire year would stimulate the production of estradiol, resulting in what would be considered a captivity artifice