Treatment of rats with dimethylnitrosamine (30 mg/kg) or diethylnitrosamine (200 mg/kg) produced a rapid increase in the activity of spermidine N1-acetyltransferase which peaked at values 7-fold greater than did control at 48 hr after exposure. This increase led to a small accumulation of N1-acetylspermidine in the liver but produced a more striking effect on putrescine which increased 30- to 40-fold after 2 days. Most of this increase appeared to be due to the conversion of N1-acetylspermidine into putrescine which is catalyzed by polyamine oxidase. Treatment with the nitrosamines also increased the conversion of spermine into spermidine which replaced the spermidine converted into putrescine. Spermine levels were therefore significantly depressed by treatment with these carcinogens. These results indicate that these hepatocarcinogens bring about an increase in putrescine and in the spermidine/spermine ratio in the liver not only by enhancement of ornithine decarboxylase but also by induction of the spermidine N1-acetyltransferase activity which is the rate-limiting step in the acetylase-oxidase pathway for interconversion of the polyamines.