Toxoplasmosis can be transmitted to mice by the introduction of Toxoplasma into the vagina. Pregnant mice were more susceptible to infection than non-pregnant animals in the ratio of 3 to 1. Obvious signs of vaginitis were not observed. Many of the infected mice remained entirely free of external signs, while a minority showed neurological or respiratory disturbances. Pregnant animals, especially those infected 6 to 10 days following conception, often died in the terminal stages of pregnancy or shortly after parturition. The possibility that the vagina may serve as one of the portals of entry of Toxoplasma in the human being and that infection may occur by sexual contact or by contamination by feces or other Toxoplasma-containing materials is discussed. The high susceptibility of the pregnant mouse to toxoplasmosis under the conditions of these experiments suggests a possible explanation for the higher incidence of congenital as compared to postnatal human toxoplasmosis and for the associated asymptomatic maternal infection. The infected but clinically normal human mothers may be compared to some vaginally infected pregnant mice which remained symptom-free.