Résumé : Tumoural islet cells of the RINm5F line were incubated for 120 min in the presence of [2-13C]propionate (10 mmol/l), and the 13C enrichment of lactate released in the incubation medium was monitored by 13C nuclear magnetic resonance. The C3/C2 ratio of resonance areas was much lower than that found with naturally 13C-enriched lactate. This reveals that symmetric Krebs cycle intermediates undergo oriented transfer in the sequence of reactions catalysed by succinate thiokinase, succinate dehydrogenate and fumarase in the mitochondria of islet cells.