The metabolism of biphenyl in the rat has been studied by using gas chromatographic and mass spectrometric methods. The free and conjugated urinary metabolites were characterized. Eight new metabolites were isolated: a dihydrodiol and two hydroxydihydrodiols were characteristic for the epoxide--diol pathway. There were two dihydroxybiphenyls, a trihydroxybiphenyl, a trihydroxymethoxybiphenyl and 4,4'-dihydroxy-3-methylthiobiphenyl. The mass spectra of the trimethylsilyl derivatives of the metabolites exhibited characteristic doubly charged and metastable ions.