Summary.-Rats were given a single dose of dimethylnitrosamine (DMN, 20 mg/kg body weight) alone or 42 or 60 hours after a non-lethal hepatotoxic dose of carbon tetrachloride (CC14) and killed 12 months later. DMN alone produced no tumours in the kidney and a few in the liver, but when given 42 hours after CC14, tumours formed in the kidneys and the number in the liver was increased. When given after 60 hours, the incidence of kidney tumours was less but that of liver tumours was further increased. A larger dose of DMN (40 mg/kg) was tolerated 42 hours after CC14 and enhanced the number of kidney and liver tumours, the latter apparently due to an increased proportion of cholangiomata. Numerous small focal proliferations of atypical liver cells and of bile duct epithelium were observed after treatment with DMN. The incidence of these lesions in the different experimental treatments varied in a similar manner to the liver tumours.

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