Administration (ip) of butadiene monoxide, a toxic metabolite of 1,3-butadiene, to rats caused the appearance of two new biliary peaks when analyzed by HPLC chromatography. These peaks were isolated and identified as the regioisomeric glutathione conjugates, S-(2-hydroxy-3-buten-1-yl)glutathione (I) and S-(1-hydroxy-3-buten-2-yl)glutathione (II), by comparison of their HPLC retention times and fast atom bombardment mass spectra to those of synthetic standards. S-(4-Hydroxy-2-buten-1-yl)glutathione, a rearrangement product formed during chemical synthesis or storage of I, was not detected. Whether butadiene monoxide was given at a dose of 14.3 or 143 mumol/kg, the amount of conjugates excreted in 30 min was at least 85% of that excreted in 120 min. Conjugate excretion in 60 min did not exhibit saturation when the butadiene monoxide dose was varied between 14.3 and 286 mumol/kg; the total amount of the butadiene monoxide dose excreted as combined I and II averaged only 7.6 +/- 4.2% (mean +/- SD, n = 12), with approximately a 3:1 ratio of isomers I:II being excreted at all butadiene monoxide doses. Whereas these results indicate a role for glutathione S-transferase-catalyzed reactions in butadiene monoxide metabolism in vivo, biliary excretion of I and II can only account for a small fraction of the butadiene monoxide dose given.