Carnitine plasma concentrations and the excretion of carnitine and individual carnitine esters were determined in 25 children and adolescents with gastrointestinal diseases receiving carnitine-free parenteral nutrition for at least 1 mo using radiochemical and radioisotopic exchange HPLC methods. Children less than 12-y-old usually had carnitine plasma concentrations less than -2 SD from the normal mean for age, whereas patients greater than 12-y-old had carnitine plasma concentrations within the normal range. Age was the only variable to correlate significantly with plasma carnitine concentrations during parenteral nutrition. Free carnitine (FC) excretion was closely correlated with plasma FC concentrations and minimal at values less than 25 mumols/L. The excretion of FC and short-chain acylcarnitines was reduced by an order of magnitude in younger compared with older patients and controls, but the excretion of "other" acylcarnitines was less affected. Some of the latter were tentatively identified using gas-liquid chromatographic and mass spectroscopic techniques as unsaturated and/or branched medium-chain carnitine esters with a carbon chain of C8-C10. The results suggest that FC and short-chain acylcarnitine are conserved by the kidney in nutritional carnitine deficiency but that there may be an obligatory renal excretion of other carnitine esters that contributes to the development of hypocarnitinemia in the younger age group.