The permeability of the vaginal mucosa for viruses in pregnant and non pregnant mice has been investigated. As a virus model, a T3-phage suspension labeled with methylene blue was inoculated into the vagina. 18 min after application of the suspension in pregnant mice, phages were detected in circulating blood by in vitro tests. In non pregnant mice, a phagemia was noted 8 min later. The rapid permeability of the vaginal mucosa for viruses during pregnancy has been substantiated by a simultaneous excretion of dye in gastric fluid and in urine. These observations may be due to the hyperemia and increased pelvic blood flow of the genital tract during pregnancy.