Studies were made on the maximal non-carcinogenic dose of dimethylnitrosamine (DMN) in rats. Groups of Wistar rats of both sexes, 6 wk old, were given standard diet without DMN (group 1), containing 0.1 ppm DMN (group 2), 1.0 ppm DMN (group 3) or 10 ppm DMN (group 4) for 96 wk and then sacrificed for hematological, serum-biochemical and histopathological examinations. After 96 wk, the weights of the body and main organs in the different groups were not significantly different. The leukocyte count and blood urea-nitrogen (BUN) in group 4 were slightly increased, but other serum findings were not significantly different in different groups. Hepatocellular carcinomas were found in group 3 (1 male and 3 females), but not in group 2. Hemangioendotheliomas of the liver, adrenal adenomas, pituitary adenomas, interstitial cell tumors of the testis, ovarian tumors and leukemia were found. Pyelonephritis was found in experimental and control animals, but no kidney tumors developed with these dose levels of DMN. On long-term oral administration to rats, 1.0 ppm DMN is the minimum carcinogenic dose, while a level of about 0.1 ppm DMN is non-carcinogenic.