The ability of furfural to induce unscheduled DNA synthesis (UDS) in hepatocytes of male and female B6C3F 1 mice and male F344 rats after in vivo administration and in vitro in precision-cut human liver slices has been studied. Preliminary toxicity studies established the maximum tolerated dose (MTD) of furfural to be 320 and 50 mg/kg in the mouse and rat, respectively. Furfural was dosed by gavage at levels of 0 (control), 50, 175 and 320 mg/kg to male and female mice and 0, 5, 16.7 and 50 mg/kg to male rats. Hepatocytes were isolated by liver perfusion either 2–4 h or 12–16 h after treatment, cultured in medium containing 3Hthymidine for 4 h and assessed for UDS by grain counting of autoradiographs. Furfural treatment did not produce any statistically significant increase or any dose-related effects on UDS in mouse and rat hepatocytes either 2–4 h or 12–16 h after dosing. In contrast, UDS was markedly induced in mice and rats 2–4 h after treatment with 20 mg/kg dimethylnitrosamine and 12–16 h after treatment of mice and rats with 200 mg/kg o-aminoazotoluene and 50 mg/kg 2-acetylaminofluorene (2-AAF), respectively. Precision-cut human liver slices from four donors were cultured for 24 h in medium containing 3Hthymidine and 0–10 m m furfural. Small increases in the net grain count (i.e. nuclear grain count less mean cytoplasmic grain count) observed with 2–10 m m furfural were not due to any increase in the nuclear grain count. Rather, it was the result of concentration-dependent decreases in the mean cytoplasmic grain counts and to a lesser extent in nuclear grain counts, due to furfural-induced cytotoxicity. In contrast, marked increases in UDS (both net grain and nuclear grain counts) were observed in human liver slices treated with 0.02 and 0.05 m m 2-AAF, 0.002 and 0.02 m m aflatoxin B 1 and 0.005 and 0.05 m m 2-amino-1-methyl-6-phenylimidazo4,5- bpyridine. This study demonstrates that furfural does not induce UDS in the hepatocytes of male and female B6C3F 1 mice and male F344 rats after oral treatment at doses up to the MTDs. Moreover, human liver slice studies suggest that furfural is also not a genotoxic agent in human liver.