Summary 1. Washed cysts of Endamoeba histolytica obtained from three human carriers known to be pathogenic to kittens were fed to albino rats (Endamoeba histolytica free) by means of a stomach tube. It was found that the infectivity for rats apparently varied with the feeding time of the animals. Thus, of 13 positives out of 78 rats used, a higher infection rate was observed in the animals on empty stomachs (6 out of 25) than when the stomachs were full (3 out of 31). By the addition of 1 per cent NaHCO3 to the feeding, 4 out of 22 rats became infected. This indicates that one of the probable mechanisms involved in the infectivity of the cysts of Endamoeba histolytica seems to be the acid concentration of gastric juice. This is analogous to what has been observed in cases of cholera vibrio. This suggests that the similar condition may occur in man and partly explain why the incidence of amebiasis is relatively low despite many possible avenues of transmission. 2. When rat to rat infection experiments were carried out with the cysts recovered from fecal pellets of rats previously infected with cysts of Endamoeba histolytica derived from a human source, thirty per cent of rats were successfully infected on the first passage. The percentage of positives was doubled on the fifth passage. This seems to suggest that during the course of successive animal passages, the human parasites have apparently become acclimatized to the intestinal conditions existing in rats. 3. Neither pathologic lesions nor diarrhea were observed in all the infected rats. Spontaneous disappearance of the infection occurred in three of 13 infected animals, though one remained a carrier till the time of autopsy. On the basis of these findings together with a comparative rarity of rodent infection in nature, one may be led to consider rats as one of the incidental rather than natural hosts in amebic infections. However, since cross infection between man and rats was apparently possible, rats may be conceived as a reservoir for Endamoeba histolytica.