The molecular mechanisms involved in the metabolic activation of certain cytotoxic furans, namely 4-ipomeanol, a natural product isolated from mouldy sweet potatoes, and 3-methylfuran (3-MF), an atmospheric pollutant, are being investigated. Not only does the environmental occurrence of certain furans have possible major toxicological significance but also there is also a growing interest in some furan derivatives as potential antitumor agents. Oxygen and NADPH dependent metabolic activation of these furans results in the formation of highly electrophilic metabolites that alkylate microsomal proteins. The pulmonary toxin, 3-MF, and 2-methylfuran (2-MF), a natural product present in cigarette smoke, coffee and many foods, are activated by microsomal nonooxidases to reactive electrophiles that bind to tissue macromolecules. Using semicarbazide (SC) as a trapping reagent, acetyl acrolein (AA) and methyl butenedial (MB) were isolated as products of microsomal oxidation of 2-MF and 3-MF, respectively. A comparison of the covalent binding of [3H]-3-MF and the amounts of MBD disemicarbazone produced in microsomal incubation in the presence and absence of NADPH and SC, revealed an inverse relationship between the two measures. NADPH dependent covalent binding of 3-MF was strongly inhibited by SC, presumably by trapping the reactive dialdehyde intermediate (MB) before it could react with tissue macromolecules. Although the initial formation of these metabolites was dependent on NADPH, the binding of synthetic 14C-AA to microsomal protein was extremely rapid and was not further enhanced by NADPH. Thus, the unsaturated aldehydes, AA and MB, appear to be the principal reactive intermediates of 2-MF and 3-MF that are bound covalently to tissue macromolecules in these preparations. Moreover, since the covalent binding of reactive material is directly correlated with toxicity, the dialdehyde is possibly responsible both for target tissue alkylation and for toxicity produced by the parent furan in vivo.