A gas chromatographic-mass spectrometric (GC-MS) procedure for the determination of stable isotope labelled glutathione has been applied to animal and human samples. The method, based on preparation of the N,S-ethoxycarbonyl methyl ester derivative of the intact peptide, is rapid and requires little or minor tissue treatment. The same method was applied to cysteine. The method was found to be reliable in terms of within-day and between-day precision, accuracy and linearity. The procedure was applied in humans and animals to determine in vivo the glutathione fractional synthesis rate using labelled cysteine infusion. The glutathione fractional synthesis rate was found to be 22.5%/day in blood from a healthy volunteer and 337+/-29%/day in rat liver.