The O-dealkylating activities in rice were investigated to obtain fundamental information relating to the selectivity and safening action of sulfonylurea herbicides by measuring 7-ethoxycoumarin O-deethylase (ECOD) and 7-methoxycoumarin O-demethylase (MCOD) activities in microsomes taken from rice shoots. NADPH cytochrome P-450 (c) reductase (NADPH cyt. c red.) activity was measured to confirm the effect of electron transfer system on cytochrome P-450 reaction. Cytochrome P-450 dependence of the reactions was demonstrated by use of safeners: 1, 8-naphthalic anhydride (NA) and ethanol; inhibitors: 1-aminobenzotriazole (1-ABT) and piperonyl butoxide (PBO); herbicides: dimepiperate, dymron, pyrazosulfuron ethyl (PSE) and bensulfuron methyl (BSM). The O-dealkylation was induced by the treatment of not only safeners (NA, ethanol) but also herbicides (dimepiperate, dymron, PSE, BSM). Furthermore, it was inhibited to varying degrees by both inhibitors (1-ABT, PBO) of induced cytochrome P-450 enzymes and by sulfonylurea herbicides (PSE, BSM). In contrast, NADPH cyt. c red. activity was not significantly changed by either induction or inhibition. These results suggest that cytochrome P-450 is involved in the oxidative detoxifying metabolism of sulfonylurea herbicides like PSE and BSM, so that these herbicides may be a substrate or competitive inhibitor of microsomal oxygenases.