1. Heteronuclear 1H/15N n.m.r. experiments are described in which 15N labelling of cellular metabolites is detected via their proton resonances. 2. These n.m.r. experiments have been used to monitor label redistribution amongst extracellular metabolites in cultures of mammalian cells incubated with L-[2-15N]glutamine, L-[5-15N]glutamine and 15NH4Cl. Label redistribution was monitored in two HeLa cell lines and in two CHO cell lines which showed a range of extractable activities of glutamate dehydrogenase, glutaminase and glutamine synthetase. 3. In cells incubated with L-[2-15N]glutamine the 15N label was subsequently found in a number of metabolites including alanine, aspartate, glycine and pyrrolidone-5-carboxylic acid. There was no detectable production of 15NH4+, showing that most of the glutamate formed in the reaction catalysed by glutaminase was subsequently transaminated rather than oxidatively deaminated by glutamate dehydrogenase. 4. Incubation of cells with L-[5-15N]glutamine showed that the ammonia in the cultures was derived predominantly from the amide group of glutamine. 5. The rate of formation of L-[5-15N]glutamine in cells incubated with 15NH4Cl was used to estimate glutamine synthetase flux in vivo. Flux in this reaction was only observable in the two CHO cell lines which express relatively high levels of the enzyme.