Ingestion of broccoli or other cruciferous vegetables inhibits the induction of cancer by chemicals and modifies some cytochrome P-450 enzyme activities. The effect of dietary broccoli on the levels of P450IA and IIB mRNA and proteins in rat liver and colon has been studied. Rats were fed a ten percent broccoli diet for 7 days. The expression of the cytochrome P-450 forms was altered to a different extent in the liver and colon. The level of total P450IA mRNA in the liver was increased by the broccoli together with the P450IA1 and IA2 proteins. Colonic P450IA1 mRNA and protein were induced by the broccoli diet, whereas only P450IA2 protein and not mRNA was detectable in colon, but the protein level was unaffected by the broccoli diet. Liver P450IIB and IIE1 proteins were increased by the broccoli diet, whereas the level of P450IIB mRNAs was not affected. In contrast, the P450IIB mRNA levels were reduced but the protein levels were increased in colon and we suggest that a feedback mechanism caused the decrease of the P450IIB mRNAs levels. Because the ratio between activation and deactivation may be an important risk determinant, we conclude that the protective effect of the broccoli diet on chemically induced tumors in rodents may be caused by the broccoli-induced changes in P450IA and IIB associated enzyme activities.