In 62 male rotogravure printers, the timeweighted average (TWA) toluene exposure during one workweek ranged from 8 to 496 mg/m 3 (median 96). Post-shift urinary excretion of hippuric acid showed a poor correlation with the air toluene concentration Level of o-cresol excretion ranged from 0 08 to 2 37 mmol/mol creatinine and was associated with the exposure (rs = 0 57, P< 0 0001), although the variation was considerable. However, this metabolite was significantly influenced by smoking habits, both in the workers ( 0 34 vs 0 10 mmol/ mol creatinine after adjustment to zero exposure for the smokers and non-smokers, respectively; P = 0 03) and in 21 unexposed controls ( 0 18 vs 0 06 mmol/mol creatinine; P = 0 002) The excretion of these metabolites was followed during vacation, when the workers were unexposed. The shared one-compartment half-time was 44 h (+ SE 30, 82) After 2-4 weeks of vacation, the concentration of o-cresol was significantly higher for the smokers than the non-smokers ( 0 14 vs 0 06 mmol/mol creatinine; P = 0.02) No smoking-associated difference was found for the urinary hippuric acid concentration However, there was an association between alcohol consumption and hippuric acid excretion (P = 0 03); no such difference was shown for o-cresol These results demonstrate that hippuric acid excretion is unsuitable for biological monitoring of toluene exposure when the exposure level is below 200 mg/m 3 Also, in spite of the favourable excretion kinetics, the impact of smoking and the large interindividual variation warrant the same conclusion for o-cresol as a means of monitoring low level exposure in an individual worker.