AbstractA newly established rat strain named TW showed congenital abnormalities in the male genital organs at about 47% incidence. These abnormalities included the unilateral or bilateral testicular hypoplasia accompanied with ipsilateral aplasia of the epididymis, ductus deferens and gland of ductus deferens. Most commonly these accessories were completely absent but in some cases the lack was partial. The other accessories such as the seminal vesicle, coagulating gland, and prostate were normal. The female genital organs were normal. The genetical analysis revealed that these defects are transmitted by multiple genes with an incomplete penetrace and a threshold effect. The TW rat provides a useful model for studying the developmental process of the male genital system.