Twenty-four yolk sac carcinomas in Lewis rats were experimentally induced by puncturing the pregnant uterine wall with a hypodermic needle at day 9-13 of gestation. Morphologically, the tumours were composed of parietal- and visceral yolk sac carcinoma and to a less degree of trophoblastic giant cells. The tumours were adapted to ascitic growth and tissue culture, but eventually became committed to express only parietal endoderm phenotype. Immunohistochemical and metabolic labelling studies on the established lines in vivo and in vitro revealed a consistent pattern with respect to their biosynthesis of the basement membrane components laminin, entactin, and type IV collagen. This model system offers a simple approach to inducing rat yolk sac carcinomas for further morphological and biochemical characterization of the basement membrane.