In newborn F344 rats, immunosuppressed with antithymocyte sera and xenografted with various human tumours (lymphomas and melanomas), regular development of nephroblastomas was observed. Transplantation experiments and chromosome analysis proved the rat origin of the tumours. The histological appearance of these rat nephroblastomas closely resemble that of the human Wilms' tumour, in which three basic components: blastema, stroma and immature epithelium can be distinguished. With serial subcutaneous-, lung-subcutaneous transplantations in immunocompetent newborn animals a spontaneously metastatic line to the lung was selected. In adult hosts, lung metastasis occurred only following orthotopic (subrenal capsule) implantation. The histogenesis and the morphological features of this rat nephroblastoma and its metastases are described.