Abstract ( 1) The effect of feeding a relatively low-protein diet containing 0.06 % DAB for 29 weeks on the activity of DAB-azoreductase, nitroreductase ( p-nitrobenzoic acid), N-oxidase ( N,N-dimethylaniline), N-demethylase (DAB), cytochrome P-450, NADPH-cytochrome c reductase, β-glucuronidase and arylsulphatase A were studied. Rapid decreases occurred in the activities of the first six enzymes, reaching minimal values at between 4 and 8 weeks. Activities then increased in all cases to control or nearly control levels. This rate of increase was least for cytochrome P-450. At 4 weeks azoreductase activity with the chemotherapeutic agent CB10-252 (I) as substrate was significantly higher than in control rats. Early increases occurred in the activities of β-glucuronidase and arylsulphatase A and the activity of the latter never dropped below the control level. ( 2) An investigation was made of the differential effects of dye feeding on some of the enzyme activities in the two major liver lobes and differences were found. ( 3) The effect of phenobarbital (PB) pretreatment on the DAB-fed rats was studied at 4-week intervals. The activities of DAB-azoreductase and of nitroreductase increased throughout the whole period, while the activities of the lysosomal enzymes were decreased. ( 4) After feeding DAB for 4 weeks the effect of PB and 3-methylcholanthrene (MC) on the activities of DAB-azoreductase, CBlO-252-azoreductase and components of the azoreductases — cytochrome P-450, NADPH-cytochrome c reductase, the CO-sensitive pathway and the NADH-dependent pathway were studied. The activity of CB1O-252-azoreductase was not induced by PB or MC, and CO did not inhibit its reduction. Its reduction depended only slightly on NADH. CO caused a greater relative decrease in the activity of DAB-azoreductase in dye-fed animals and also in animals following PB and MC pretreatment, implying a greater role of cytochrome P-450 in dye-fed animals.