The effect of the food additive butylated hydroxyanisole on the nitrosation of dibutylamine was studied in vitro and in vivo, in rats. At the highest concentration tested, butylated hydroxyanisole significantly inhibited the in vitro formation of N-nitrosodi-n-butylamine from dibutylamine and nitrite by 35%. This effect was not observed in animals given dibutylamine, NaNO2 and butylated hydroxyanisole by gavage in the same ratio that produced an effect in vitro. The oral administration of butylated hydroxyanisole to rats given 0.5% nitrate in the drinking water and dibutylamine (0.38 mmol/kg 3 times/day p.o.) was also with no effect, although the experimental model used proved suitable for studying the modulating effect on endogenous nitrosation of different chemicals such as ascorbic acid or potassium thiocyanate. The discrepancy between in vitro and in vivo results is discussed.