Although the pathogenesis of cancer of the kidney in man is largely unknown, certain aspects of renal carcinogenesis in experimental animals have now been elucidated.<sup>2</sup>Three main agents have been studied—hormones, chemical carcinogens, and viruses—and the present account will be confined to a consideration of these factors. Spontaneous renal neoplasms are rare in laboratory animals,<sup>2</sup>but the incidence may rise as a result of inbreeding, both in mice and in rats. The exact patterns of genetic transmission are obscure, except for the Wistar rats, studied by Eker and Mossige,<sup>3</sup>in whom the tumors appeared to be inherited through a single dominant gene which was not sex-linked. The incidence of renal tumors in rats and mice is also increased if animals are exposed to ionizing radiations,<sup>4,5</sup>although observations of this kind inevitably beg the general question of "spontaneous" neoplasia. The carcinogenic effect of estrogens on the kidneys of