The expression, inducibility, and regulation of four different cytochrome (cyt.) P-450 isoenzymes (PB1, PB2, MC1, and MC2) NADPH-cytochrome P-450 reductase, the glutathione transferases (GSTs) B and C and microsomal epoxide hydrolase (mEHb) have been studied during nitrosamine-induced hepatocarcinogenesis using immunohistochemical techniques. The investigations revealed basic differences in the expression of the individual drug metabolizing enzymes in the course of neoplastic development. While the two GSTs and mEHb were increased in all preneoplastic and benign neoplastic lesions, the levels of the distinct cyt. P-450 isoenzymes were characteristically different from each other. Following initial changes in the expression of these enzymes in early preneoplastic lesions (i.e., increase of cyt. P-450 PB1 versus slight decrease of the other cyt. P-450 isoenzymes), a continuous reduction of all cyt. P-450 isoenzymes was observed during the further course of hepatocarcinogenesis. In progressed neoplastic nodules, all cyt. P-450-isoenzymes and NADPH cyt. P-450 reductase were decreased to varying extents. Treatment of animals with inducers of the monooxygenase system, such as phenobarbital, 3-methylcholanthrene and polychlorinated biphenyls, led to a rather heterogenous pattern of enzyme alterations in preneoplastic and neoplastic lesions. Following administration of phenobarbital, some islets responded to the same degree as the surrounding tissue, others were less or not at all inducible and a few of the lesions showed a prominent increase in cyt. P-450 PB2 and NADPH-cyt. P-450 reductase levels. The interesting finding that these two enzymes always showed concurrent changes may be indicative of a common regulation.(ABSTRACT TRUNCATED AT 250 WORDS)