The depletion of glutathione (GSH) reported in very-low-birth-weight infants is implicated in several pathologies, especially if deficiency occurs during foetal development. The cause of this depletion is suggested to be modification of GSH turnover. To probe the role of GSH, a reliable non-invasive method adapted to very-low-birth-weight infants is required. In this paper, we report the preparation of the N,S-ethoxycarbonyl methyl ester derivatives of GSH and glycine and their application to the measurement of C-13/C-12 ratios at natural abundance in erythrocyte samples by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The technique allowed the determination of C-13/C-12 ratios at natural abundance with a precision < 3% and within-day and between-day variabilities both < 4%. The method is able to determine accurately low C-13-enrichments in GSH (0.00241 to 0.00753 Atom Percent Excess) in erythrocyte extracts following incubation with C-13-glycine at low specific enrichment (approx. 1.5 atom %). Excellent agreement was obtained between the calculated GSH fractional synthesis rate (FSR) in human adult blood (approx. 300% day(-1)) using the low-enrichment C-13-glycine/GC/C/IRMS protocol and that using highly enriched C-13-glycine (99 atom %)/GC/MS with the same derivative. The GC/C/IRMS method was shown to be suitable to measure the in vitro GSH FSR (200-660% day(-1)) in human venous and arterial blood from the umbilical cord. This approach provides a good tool for studying the turnover of GSH in vitro in infants, allowing both the use of minimal amounts of tracer and negligible perturbation of endogenous precursor pools