The activities of liver drug-metabolising enzymes for 16 aromatic or chlorinated hydrocarbons were measured in male rats after a three-week daily intake of ethanol amounting to 30% of total energy intake. Although the ethanol feeding produced only a slight increase in the microsomal cytochrome P-450 content, it increased the in-vitro metabolism of most hydrocarbons three-to six-fold. That a major part of this enhanced activity disappeared after one-day withdrawal of ethanol suggests that recent intake of ethanol plays an important part in accelerating the metabolism of hydrocarbons. The enzyme activity enhanced by ethanol was found to be related with changes occurring not in the soluble but in the microsomal fractions. A metabolism study using toluene as a model substrate indicated that chronic ethanol consumption increases the in-vivo metabolism of this hydrocarbon in rats.