The virgin female laboratory rat has been studied as a model of the ecosystem represented by the vaginal mucosa and its bacterial flora. Gram-stained vaginal smears abtained from rats having normal estrous cycles showed cyclic variation in the number of bacteria present. Viable plate counts of material obtained by vaginal lavage demonstrated daily changes in the density of the bacterial population of the vagina in relation to the estrous cycle. Bacterial counts observed during the estrus phase of the cycle in individual rats were 10,000 to 100,000 times greater than viable counts observed during the metestrus or diestrus phases of the cycle. These studies represent the first reported demonstration that physiologic changes occurring during the estrous cycle control the size of the bacterial population of the vagina.