The authors carried out this study according to three parameters (dose, level, age of the rats and way of administration) on the action of fluor study in liver metabolism of dimethylnitrosamine and benzo(a)pyrene. The results showed that, in parallel with the increase of the dose level, at certain concentrations, it occurs generally an induction of dimethylnitrosamine-demethylase and a decrease in the amount of cytochrome P450 either by intraperitoneal injection, by ingestion or by inhalation. On the other hand, fluoride does not affect benzo(a)pyrene metabolism. Moreover, inhalation of FH looks the most suitable for this study. It allows to keep a relatively constant rate of F' in the blood and bring about more important changes in the metabolism of dimethylnitrosamine (30 to 70% induction) and in the amount of cytochrome P450 (10 to 40% decrease), as compared to the controls. The results suggest that, in the conditions of our experiments, fluoride would exert an favourable effect on the carcinogenic power of dimethylnitrosamine without affecting benzo(a)pyrene one.