Epidemiological and laboratory studies suggest that dietary broccoli may prevent or delay a variety of cancers. Broccoli and other crucifers contain a relatively unique family of secondary metabolites called glucosinolates. Glucoraphanin, the major glucosinolate in broccoli, is hydrolyzed by an endogenous plant myrosinase to form either the potent anticarcinogen sulforaphane (SF) or sulforaphane nitrile (SF nitrile). The bioactivities of SF and SF nitrile were compared in rats and in mouse hepatoma cells. Male, 4-week-old, Fischer 344 rats were administered SF or SF nitrile (200, 500, or 1000 micromol/kg) by gavage daily for 5 days. Hepatic, colonic mucosal, and pancreatic quinone reductase and glutathione S-transferase activities were induced by high doses of SF, but not by SF nitrile. When Hepa 1c1c7 cells were exposed to increasing levels of each compound for 24 h, quinone reductase showed a 3-fold maximal induction over control at 2.5 microM SF and a 3.5-fold maximal induction over control at 2000 microM SF nitrile, the highest concentration tested. These results demonstrate that SF nitrile is substantially less potent than SF as an inducing agent of phase II detoxification enzymes. Therefore, glucoraphanin hydrolysis directed toward the production of SF rather than SF nitrile could increase the potential chemoprotective effects of broccoli.