The contribution of Phase II conjugation reactions to human disposition of 2-amino-3,8-dimethylimidazo-[4,5-f]quinoxaline (MelQx) was investigated by analysis of urine for MelQx and its sulfamate and glucuronide metabolites. Subjects consumed pan-fried fish, beef, or bacon and collected 0-12 and 12-24-h postconsumption urine samples. MelQx content of the samples was determined both with and without acid treatment that quantitatively hydrolyzes the Phase II conjugates. The amount of unconjugated MelQx in the urine of seven subjects ranged between 2 and 36 ng in the first 12-h sample and was undetectable in the second. Hydrolysis increased the MelQx content 3-13-fold in the urine of six subjects, while the urine of one subject showed no significant change. Unconjugated MelQx excreted in urine was found to range between 0.5 and 4.7% of the ingested dose. In acid-treated urine the amount of MelQx was found to range between 1 and 14% of the ingested dose. A method for isolating the acid-labile conjugates in urine was developed, which included the following steps: acetone/methanol precipitation; solid-phase extraction; ion exchange fractionation, normal phase aminopropyl fractionation, and reverse phase high pressure liquid chromatography separation of the metabolites. Acidic hydrolysis of the fractions obtained in the last step, followed by gas chromatography-mass spectrometry analysis of the MelQx produced, was used to confirm the presence of the sulfamate and the glucuronide metabolites in human urine. The results provide evidence that glucuronidation and amine sulfamation are significant pathways of detoxification of MelQx in humans. In addition, the increased amount of MelQx released after acid hydrolysis facilitates the quantitative analysis of urinary MelQx.