Abstract
Histological variations of canine deciduoma which was induced in the non pregnant horn at several stages of unilateral pregnancy were examined. In the first half of the unilateral pregnancy, deciduoma was characterized by the cystic glandular hyperplasia corresponding to each of the stages in normal early placentation. In the second half, deciduoma could not be induced and few histological reactions were recognized. The endometrium looked normal for late diestrus with no growth of the uterine glands. These differences might reflect the latent strength of the uterine glands to proliferate and dilate in the stimulated periods.