Physiological effects of furfural on Saccharomyces cerevisiae growing on ethanol (15 g.l(-1)) or acetate (20 g.l(-1)) as the carbon and energy source were investigated. Furfural (4 g.l(-1)) which was added during the exponential growth phase in batch cultures, was found to strongly inhibit cell growth on both carbon sources. No biomass formation occurred in the presence of furfural. However, furfural was in both cases converted to furfur;FI alcohol and furoic acid, and growth resumed after complete conversion of furfural. During growth on ethanol, a rapid initial conversion of furfural to furfuryl alcohol was observed during the first few minutes after the addition of furfural, after which the conversion rate decreased to approximately 0.15 g.g(-1).h(-1) for the remaining conversion time. Acetaldehyde accumulated in the medium during the first few hours of conversion. Interestingly, addition of acetate after furfural addition resulted in an increased conversion rate of furfural and a higher carbon dioxide evolution rate, but no growth was observed until after complete conversion of furfural. Furfural addition to cells growing on acetate as the sole carbon source induced no formation of acetaldehyde, and the furfural conversion rate was lower than that on ethanol. The relationship between inhibition effects of furfural and NADH consumption is discussed.