The metabolic activation of (S)-5,6-difluoro-4-cyclopropylethynyl-4-trifluoromethyl-3,4-dihydro-2(1H)-quinazolinone, DPC 963, in rats was investigated by identifying and characterizing the GSH and mercapturic acid conjugates excreted in the bile and urine, respectively. The structures of these adducts, which were unequivocally elucidated by LC/MS/MS and NMR experiments, revealed the existence of at least three distinct metabolic pathways leading to these products. One of the pathways, which has been described previously, involves the activation of the acetylene group after an initial hydroxylation on the methine carbon of the cyclopropyl ring. Metabolite M1 was demonstrated to be formed via this pathway after an enzymatic addition of GSH across the triple bond of the substituted acetylene. The second pathway, also previously described, leads to diastereoisomeric GSH adducts M3 and M4 after the formation of a highly reactive oxirene intermediate. This postulated oxirene subsequently rearranges to an alpha, beta-unsaturated cyclobutenyl ketone intermediate capable of undergoing a 1,4-Michael addition with a nucleophile such as GSH. In addition to these pathways, DPC 963 was found to undergo a metabolic activation previously undescribed for structural analogues of this compound. It is postulated that an oxidative defluorination mediated by cytochrome P450 leads to the formation of a putative benzoquinone imine intermediate which subsequently reacts with GSH to form two aromatic ring-substituted regioisomeric conjugates, M5 and M6. In addition to forming the GSH adducts, the benzoquinone imine was also found to be reduced to its unreactive hydroquinone metabolite, which was excreted as the glucuronide conjugate in rat bile. Studies with induced rat microsomes, cDNA-expressed rat P450 isozymes, and polyclonal antibodies against rat P450 clearly demonstrated that the rat P450s 3A1/3A2 were responsible for the formation of postulated oxirene and benzoquinone intermediates.