Abstract The inclusion of an SH-trapping compound in the radioenzymatic carnitine method based on the conversion of radioactive acetyl-CoA to acetylcarnitine in the presence of carnitine acetyltransferase results in linear standard curves. N-Ethylmaleimide was preferred since the amount of free carnitine was less overestimated when acetylcarnitine was present compared to tetrathionate. Urinary excretion of carnitine and its derivatives was determined in 13 healthy fullterm boys. On Days 3 and 6 after delivery, total carnitine excretion was similar, 7.5 μmol/24 hr ± 0.90 (SE) and 7.0 μmol 24 hr ± 1.16 (SE) despite the ingestion of more breask milk on Day 6. Regarding the distribution of free, acyl, and total carnitine, the variation between individuals was large. Weak but significant differences were noted pointing at a higher degree of carnitine acylation on Day 3. It was shown by a comparison of these variables between the newborns and healthy boys, 8–15 years, that the newborns had a markedly increased acylation degree of carnitine. The changes may be consistent with the fact that lipids are the major energy source in the neonatal period.