The dynamic changes of plasma acylcarnitine levels in 1- to 7-y-old children during fasting and after the ingestion of sunflower oil were studied. Glucose, 3-hydroxybutyrate, acetoacetate, FFA, and individual plasma acylcarnitine levels were monitored in both conditions. Fasting experiments lasted for 20 h, and acylcarnitine concentrations were analyzed at 0, 15, and 20 h of fasting. During the fat load, acylcarnitine levels were analyzed at 0, 60, 120, and 180 min. In both tests, a generalized increase of all plasma straight-chain acylcarnitines was observed. Acetylcarnitine contributed the most to the increase of total esterified carnitine. In addition, we demonstrated that the relative increase of each individual acylcarnitine during enhanced fatty acid oxidation is tightly related to its molecular structure and chain length. Fasting as well as the fat load primarily resulted in an increase of unsaturated acylcarnitines. During fasting, C(12:1) and C(14:1) showed a relatively high increase, whereas after the fat load C(16:2) and C(14:2), metabolites of linoleic acid (66% of the fat load), were the main acylcarnitines that increased.