PURPOSE. An earlier study showed that 4-hydroxynonenal (HNE), formed as a result of increased lipid peroxidation in oxidative stress, causes loss of lens transparency. To determine how HNE is detoxified in ocular tissues, its metabolism in cultured human lens epithelial cells (HLECs) as well as rat lens was investigated. METHODS. Rat lens or HLECs were incubated with 30 nmol (5 × 105 cpm/μmol) of HNE in 2 mL Krebs-Hansleit buffer for 1 hour at 37°C. The medium, after ultrafiltration was analyzed by high performance liquid chromatography (HPLC), using a C-18 reversed-phase column. The metabolites were separated by using a gradient consisting of solvent A (0.1% aqueous trifluoroacetic acid) and solvent B (100% acetonitrile) at a flow rate of 1 mL/min. Fractions containing radioactivity were pooled and analyzed using electrospray ionization mass spectroscopy (ESI-MS) or gas chromatography-chemical ionization mass spectroscopy (GC/CI-MS). RESULTS. On HPLC, the incubation media from cultured lens and HLECs separated into three major radioactive peaks. Peak I of the HLECs and lens treated with HNE was identified to be a mixture of glutathione (GS) conjugates of HNE and 1,4-dihydroxy-2-nonene (DHN). The identity of the conjugates was confirmed by ESI-MS. Based on the retention times, peaks II, and III were assigned to 4-hydroxy-2-nonenoic acid (HNA) and unmetabolized HNE, respectively. The identities of HNA and HNE were confirmed by spiking the tissue extracts with synthetic metabolites and finally by GC/CI-MS. Sorbinil, an aldose reductase (AR) inhibitor, attenuated GS-DHN levels and cyanamide, an aldehyde dehydrogenase inhibitor, decreased formation of HNA. CONCLUSIONS. The results show that the major metabolic transformation of HNE in rat lens and HLECs involves conjugation with GS and oxidation to HNA. The GS-HNE conjugate is reduced to GS-DHN by AR. Thus, under normal physiological conditions, the lens has multiple routes to detoxify HNE. However, oxidative stress may overwhelm the metabolic capacity of the lens to detoxify HNE and lead to formation of cataract.