Histologic and ultrastructural features of nephroblastomas in Sprague-Dawley rats induced by a single transplacental injection of ethylnitrosourea (ENU) on the 15-16th day of gestation are described and their significance is discussed with respect to the histogenesis of the tumors. Many renal tumors were observed, in addition to predominant occurrence of tumors of the nervous system. Forty-two of 142 offsprings survived over 100 days developed renal tumors, of which 37 rats had nephroblastomas, three rats had cortical adenomas, and one rat had a cystadenoma. The nephroblastomas were found in 35 rats unilaterally and in two rats bilaterally. Most of these tumors had various amounts of hyalinized stroma. In the kidneys with nephroblastomas, isolated, small foci of renal blastema were also occasionally noted. Electron microscopy revealed that nephroblastomas consisted of two types of cells, namely, epithelial and mesenchymal cells. The epithelial cells showed cellular differentiation of varying degree; they are freely lying in the stroma, forming well differentiated tubules or immature tubule-like structures and solid cell nests. Most of the undifferentiated cells observed around the tubular structures and cell nests are demonstrated to have epithelial features. These observations seem to support the concept that nephroblastoma is originated from metanephric blastema.