Sulfate conjugation of benzo[alpha]pyrene(BP) metabolites and derivatives was studied. The reaction sequence consisted of two steps; activation of sulfate ion to 3'-phosphoadenosine-5'-phosphosulfate and transfer of the activated sulfate to the BP-derivatives. Both reactions were carried out by enzymes located in the rat liver 105 000 g supernatant. The reactions required MgCl2. Phenol and quinone derivatives were generally good substrates for sulfate conjugation and different reactivities were observed with the dihydrodiol derivatives. Sulfate conjugates were more polar than their parent BP-derivatives and except for quinone conjugates were easily extracted with ethyl acetate. The role of sulfate conjugation in BP carcinogenesis is discussed.