The physiological effects of 5-hydroxymethylfurfural (HMF) on Saccharomyces cerevisiae CBS 8066 in the presence and absence of furfural were studied. Experiments were carried out by pulse addition of HMF (2-4 g/l) as well as HMF (2 g/l) together with furfural (2 g/l) to batch cultivations of S. cerevisiae. Synthetic medium with glucose (50 g/l) as carbon and energy source was used. Addition of 4 g/l of HMF caused a decrease (approx. 32%) in the carbon dioxide evolution rate. Furthermore, the HMF was found to be taken up and converted by the yeast with a specific uptake rate of 0.14 (+/- 0.03) g/g . h during both aerobic and anaerobic conditions, and the main conversion product was found to be 5-hydroxymethylfurfuryl alcohol. A previously unreported compound was found and characterized by mass spectrometry. It is suggested that the compound is formed from pyruvate and HMF in a reaction possibly catalysed by pyruvate decarboxylase. When HMF was added together with furfural, very little conversion of HMF took place until all of the furfural had been converted. Furthermore, the conversion rates of both furfural and HMF were lower than when added separately and growth was completely inhibited as long as both furfural and HMF were present in the medium.