Furan is a liver toxicant and carcinogen that occurs in heat-processed foods. Due to its volatility, analysis of furan in food does not provide reliable estimates of exposure. Biomarker-based approaches offer the opportunity to more accurately assess human exposure, but a correlation between concentrations of potential biomarkers of furan exposure and external dose has not been established. Bioactivation of furan and subsequent reaction of cis-2-butene-1,4-dial (BDA) with cellular nucleophiles gives rise to a range of metabolites that may serve as biomarkers of furan exposure. In this study, N-[4-carboxy-4-(3-mercapto-1H-pyrrol-1-yl)-1-oxobutyl]-L-cysteinylglycine cyclic sulfide, a mono-glutathione adduct of BDA (GSH-BDA), and R-2-acetylamino-6-(2,5-dihydro-2-oxo-1H-pyrrol-1-yl)-1-hexanoic acid, an adduct of BDA with Nα-acetyl-L-lysine (NAcLys-BDA), were synthesized and analysed by LC-MS/MS in urine of rats treated with furan at 0, 0.1, 0.5 and 2.0 mg/kg bw for 5 and 28 days. GSH-BDA and NAcLys-BDA were both excreted in a dose-related manner. 24 h excretion rates ranged between 0.6 and 1.1% of the administered dose for GSH-BDA, and 1.4-2.1% for NAcLys-BDA. In contrast to GSH-BDA, NAcLys-BDA was also present in urine of controls, suggesting either endogenous formation or background exposure. Overall, the close correlation between urinary furan metabolites and external dose provides experimental support for biomarker-based approaches to monitor human exposure to furan.