The metabolism and excretion of intramuscularly administered 14C-glutathione conjugates of styrene oxide (Ia and IIa) were studied in the winter flounder at three dose levels. The various radiolabeled thioether metabolites excreted were separated by reverse-phase high pressure liquid chromatography, and identified and quantitated by cochromatography with synthetic standards. The urine was the major excretion route for radioactivity derived from the glutathione conjugates (up to 90%) at each dose level (1.0, 3.9, and 24.4 mg/fish) studied. The corresponding cysteine derivatives (Ic, IIc) were the major urinary metabolites although the N-acetylcysteine derivatives (Id, IId), or mercapturic acids, were also present in significant amounts at each dose and excretion interval examined. Unchanged glutathione conjugates of styrene oxide were the major radioactive constituents of 24-hr bile samples from the treated flounder, although significant amounts of the cysteinylglycine (Ib, IIb), cysteine, and N-acetylcysteine derivatives were also present in bile. Bile was a minor excretory route relative to urine. The oxidation of 14C-styrene to styrene 7,8-oxide by the cytochrome P-450-dependent monooxygenase system of hepatic microsomes of winter flounder was also demonstrated; likewise, styrene was converted to the diastereomeric glutathione conjugates of styrene 7,8-oxide by 9,000g supernatant fractions of flounder liver supplemented with glutathione. This study demonstrated that flounder liver can convert styrene to glutathione conjugates to styrene oxide and that mercapturic acid biosynthesis occurs after parenteral administration of a xenobiotic-glutathione adduct to this marine species, although the major urinary metabolites were the cysteine conjugates rather than the anticipated mercapturic acid derivatives.