This study followed the metabolism of [13C]glucose under anaerobic and aerobic conditions in the adult filarial nematodes Brugia pahangi and Dipetalonema viteae using non-invasive 13C nuclear magnetic resonance techniques. Adult B. pahangi and D. viteae showed a rapid uptake of labelled glucose which remained linear over at least 4 h. Both species of worm removed significantly more glucose from the medium under aerobic conditions than under anaerobic conditions. The principal product of metabolism, under both anaerobic and aerobic conditions, was lactate, which accounted for 62-71% of the original [13C]glucose. Examination of the maintenance medium following worm incubation revealed a further excretory product which was identified as succinate. This product accounted for 1-2% of labelled glucose in adult B. pahangi and 2-5% in adult D. viteae. The presence of succinate as an excretory product suggests that a partial reversed tricarboxylic acid cycle is active in these filarial nematodes. A further peak was identified in the worm homogenate and identified as trehalose. The disaccharide was not an excretory product and occurred only within the worm. The peak accounted for 13-14% of the 13C-labelled glucose in B. pahangi and 15-16% in D. viteae. Trehalose has not been previously recorded in either of these nematodes and is likely to have a storage function.