We examined lactate dehydrogenase (LDH) and glucose-6-phosphate dehydrogenase (G6PDH) activities and citrate levels in 37 human brain tumours and 10 normal brain tissues. All tumours showed decreased LDH activities. Since tumour cells rely on the energy produced either glycolysis (if glutamine is limited) or glutaminolysis (if carbohydrates are limited), we conclude that the source of decreased LDH activities can be high glutamine concentrations. In general the citrate levels of tumours were higher than those of normal brain. Previous studies have indicated that citrate carbons were mainly rerouted to synthesis of cytoplasmic lipids rather than to oxidation in the Krebs cycle. Thus high citrate levels of tumours may be a significant source of enhanced lipid synthesis that is needed for increased membrane synthesis. We found that G6PDH activities were highly variable in tumours, and suggest that these cannot reflect their proliferative activity. (C) 1998 Chapman & Hall Ltd.