Rat vaginal epithelial layers from animals in different phases of the estrous cycle showed positive immunofluorescence when treated with either monoclonal antibody to intermediate filaments or immunoglobulin G fraction of antiserum raised against epidermal keratin filaments. During estrus, the intensity of fluorescence observed was maximum in the keratinized cellular layers. In estradiol-primed immature and ovariectomized rats the maximum fluorescence intensity was observed in the layers immediately lining the lumen. However, basal layers in ovariectomized rats also showed some fluorescence. Data presented in this communication indicate that the abundance of keratin filaments in vaginal epithelial cells can be modulated by altering the level of estradiol in the system.