N-Nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) are widely occurring nitrosamines and require enzyme-catalyzed activation for their carcinogenic actions. The low Km forms of the enzyme are generally considered to be important in the activation of environmental carcinogens. In this work we examined the role of cytochrome P450IIE1--a constitutive enzyme that is also inducible by acetone, ethanol, fasting and other factors--in catalyzing the dealkylation and denitrosation of these two carcinogens. The experimentally determined Km value of NDMA demethylase depended upon the experimental conditions and was lower when lower protein concentrations were used. Low Km values of 15-20 microM were observed for NDMA demethylase with different preparations of microsomes. In the deethylation of NDEA, a low Km of approximately 40 microM was observed for both control and acetone-induced microsomes. Immunoinhibition studies indicated that P450IIE1 was responsible for almost all the low Km NDMA demethylase activity in acetone-induced microsomes and greater than 80% in control microsomes. This enzyme was also responsible for about three-quarters of the low Km NDEA deethylase activity in acetone-induced microsomes and about half in control microsomes. The denitrosation of NDMA and NDEA was inhibited to approximately the same extents as the dealkylation reactions under different experimental conditions, suggesting the involvement of the same enzyme and perhaps a common initial intermediate in these two types of reactions. The relevance of this work and its relationship to related information in the literature are discussed.