Two experiments of renal carcinogenesis with dimethylnitrosamine (DMN) are described. In experiment I, the carcinogenicity of a single neonatal injection of DMN was compared in 34 intact rats and in 38 rats right-nephrectomized at 8–10 weeks of age. A total of 37 tumor-bearing kidneys were seen: both tubular and anaplastic; tumor incidence and time of observation showed no significant differences between intact and nephrectomized animals. Liver tumors were seen in 38 of 72 rats. In experiment II, 64 adult rats were given DMN in the drinking water for 2 weeks at a concentration corresponding to half an effective dose: 20 rats were used as non-nephrectomized controls; the others were right nephrectomized either on the 7th - 8th day of treatment with DMN or 15 days prior to the beginning of the treatment with the carcinogen. In no case did unilateral nephrectomy have a modifying effect upon carcinogenesis. In nephrectomized rats nonneoplastic nephritic changes were more severe than in intact animals. The design of the present experiments is compared with that of previous studies on the role of either partial hepatectomy or unilateral nephrectomy in carcinogenesis.