The distribution of trans-4-hydroxy-2-hexenal (t-4HH), a pyrrolizidine alkaloid metabolite and a lipid peroxidation product, was studied following the injection of [3H]t-4HH into the hepatic portal vein of male Sprague-Dawley rats. Less than 3% of the tritium label remained in the liver 24 hr after administration with levels in the other major organs correspondingly lower. The majority of recovered radioactivity (77-83%) appeared in the urine with 60-69% appearing within 8 hr after administration. Most of the urinary radioactivity (75%) was recovered in an acidic fraction following acid/base extraction. A tandem mass spectrometry technique using negative ion fast atom bombardment (FAB) ionization in combination with Mass-analyzed Ion Kinetic Energy Spectrometry verified that a C-3 mercapturic acid conjugate of trans-4-hydroxy-2-hexenal was produced as a urinary metabolite. This compound presumably forms via a Michael addition of glutathione at C-3 of t-4HH followed by hydrolysis of glutamate and glycine and acetylation to yield the mercapturic acid. While Michael additions of glutathione to t-4HH-related compounds have been observed in vitro, these results provide in vivo evidence of this mechanism.