Microsomes obtained from different groups or rats, following pretreatment with different dosages or 3,4-benzpyrene (BP), aflatoxin Bl (Bl), ethionine (ET) and/or a combination of these, were used to obtain initial velocity data for BP disappearance. The values for Km, Vmax and K/v for different groups were obtained by the computer analysis of initial velocity data. Vmax and Km values were used to assess the quantitative and qualitative changes of microsomal B:2 hydroxylase, respectively, caused pretreatment. BP pretreatment greatly enhanced the V and also decreased the Km values. The increase in Vmax paralleled the increase in the dosage of BP from BP(lX) to BP(3X). When ET, a protein inhibitor, was given in conjunction with BP it blocked the increase in Vmax but did not block the effect of BP on the Km. The effect of BP on the qualitative properties (Km) of enzyme was observed to be a slow time dependent phenomenon which required the simultaneous presence of both the enzyme and the inducer. / Ph. D.