The outcome for children with inherited disorders of propionate metabolism is poor. To facilitate development of improved treatment in these conditions, we described techniques for the estimation of the rate of production and removal of propionate in vivo. Propionate turnover was determined in 4 healthy adults using continuous infusions of sodium [2H5]propionate and sodium [13C]propionate. The mean fasting plasma propionate concentration measured by a sensitive technique employing high performance liquid chromatography, following a two-stage extraction procedure and derivatisation with bromophenacyl bromide, was 3.3 mumol/l (SD 0.5). The isotopic enrichment of the bromophenacyl propionate derivative was measured by gas chromatography/mass spectrometry and mean propionate turnover was calculated to be 17.6 mumol/kg per h (SD 5.9). These methods allow rapid (less than 3 h) assessment of propionate turnover in man and are suitable for application in children with inherited disorders of propionate metabolism.