In female voles that are sexually quiescent, both those that have not yet attained breeding maturity, and those that have undergone regression after attainment of sexual maturity, the vaginal orifice is not evident. The canal is sealed externally by a membranous layer of epithelium. Presence of a vaginal orifice indicates that the individual is in some active stage of the breeding cycle. The appearance of the orifice varies between different females, and it changes in the same female from day to day or even from hour to hour. Presumably these changes in the vaginal orifice are cyclical and are closely correlated with oestrus, but attempts to trace them were unsuccessful largely because the normal cycle was rapidly suppressed in captive voles, which soon became sexually quiescent. Individual voles living under natural conditions were not trapped with sufficient regularity to permit tracing the details of changes in their genitalia. In those females having the vaginal orifice most developed, the margins are turgid and slightly inflamed. The circular opening gapes 1.0 to 1.5 mm. in diameter when the tail is raised. A female may remain in this condition for two days or more. Vaginal smears at this stage often showed nucleated cells characteristic of oestrus. Subsequently the margins of the orifice become less prominent and the opening becomes smaller. The dorsal and ventral walls adhere until an opening is no longer evident unless the adjacent skin is stretched. In pregnancy the orifice is occasionally sealed, but usually is evident. It is, however, less prominent than in oestrus, and does not gape. The margins are less turgid than in oestrus, and the opening is in the form of a transverse slit through which the purplish epithelial lining of the dorsal wall of the vagina can be seen. After parturition, placentae and bloody discharge often are in evidence in the vaginal canal. Females that have not given birth to young recently may also have bloody mucous discharge. Its significance has not been determined. In females that are undergoing sexual regression, the margins of the vaginal orifice become shrunken and pale, and the orifice becomes partly or wholly sealed.