Following administration to rats of a single ip dose (6.6 mg kg-1) of the investigational antitumor agent caracemide (N-acetyl-N,O-bis[methylcarbamoyl]hydroxylamine), the mercapturic acid derivative N-acetyl-S-(N-methylcarbamoyl)cysteine (AMCC) was identified in urine by thermospray LC-MS. Quantification of this conjugate was carried out by stable isotope dilution thermospray LC-MS, which indicated that the fraction of the caracemide dose recovered as AMCC in 24-h urine collections was 54.0 +/- 5.5% (n = 4). Since AMCC is known to represent a major urinary metabolite of methyl isocyanate (MIC) in the rat, the results of this study support the contention that caracemide yields MIC as a toxic intermediate in vivo. Furthermore, with the aid of a specifically deuterium-labeled analog of caracemide ([carbamoyloxy-C2H3]caracemide), it was shown that the methylcarbamoyl group of AMCC derived from both the O-methylcarbamoyl (72%) and N-methylcarbamoyl (28%) side chains of the drug. In view of these findings, it is concluded that caracemide acts as a latent form of MIC in vivo and that this reactive isocyanate (or labile S-linked conjugates thereof) may contribute to the antitumor properties and/or adverse side-effects of caracemide.