Microsomes isolated from hyperplastic liver nodules and hepatomas, induced by DL-ethionine, exhibited a reduced cytochrome P-450 content and aminopyrine N-demethylase activity when compared to the organelles of control and surrounding non-nodular liver. Phenobarbital administration to rats caused an increase of microsomal protein, cytochrome P-450 and aminopyrine N-demethylase in all tissue tested. In the hepatoma the rise of cytochrome P-450 and aminopyrine N-demethylase/g of tissue was very low and it is compensated by a slight increase of microsomal protein. In hyperplastic nodules as well as in control and surrounding livers, cytochrome P-450 and aminopyrine N-demethylase increased more than microsomal protein. However, the phenobarbital-induced stimulation was significantly lower in hyperplastic nodules than in control and surrounding livers.