Unlike those experiments described in the previous several reports, a carcinogen was given to pregnant rats in our experiment to induce a tumor similar to yolk sac tumor which often occurs in human cases, aiming at contribution to establishment of an experimental model. Using Wister rat on the 10th day of gestation, ater removal of the fetus, the yolk sac was extracted out of the uterus, and a carcinogen was administered to it. Induced tumor was examined morphologically. By the 16th week after experiment, tumor was induced in 70% out of rats treated. The tumor volume was varied from a bean to a thumb tippize. Histologically, most of them were considered to be teratoma. But in 30% of them, human pure-type yolk sac tumor or the tumor similar to it were observed. Reticular and endodermal sinus or polyvesicular vitelline patterns were also clearly observed. Presence of hyaline body by PAS-staining and AFP by fluorescent antibody method were confirmed.