Linoleic acid metabolism was studied during the first week of life in 10 breast-fed, full-term infants. Uniformly 13C-labeled linoleic acid (1 mg/kg body weight) was given orally. The 13C content was determined in expired CO2 over 6 h and in plasma phospholipid fatty acids over 3 d. Total CO2 production determined by indirect calorimetry was 16.7 +/- 10.6 mL/min (mean +/- SE). Over 6 h 7.4 +/- 0.6% of the ingested 13C-labeled linoleic acid was oxidized to CO2. Plasma phospholipid linoleic acid showed maximal 13C enrichment 24 h after tracer application (delta over baseline 178 +/- 24/1000). Enrichment of dihomo-gamma-linolenic acid increased from d 2 to d 5 of life (p < 0.002), with delta over baseline values of 2.1 +/- 0.5/1000 at 24 h, 3.7 +/- 10.9/1000 at 48 h, and 4.4 +/- 1.0/1000 at 72 h. 13C content of arachidonic acid tended to increase insignificantly. Areas under the curve of plasma tracer concentration over time were calculated for plasma n-6 phospholipid fatty acids. Percentages of total areas under the curve of the investigated n-6 fatty acids were 97.3 +/- 0.8% for linoleic acid, 1.5 +/- 0.6% for dihomo-gamma-linolenic acid, and 1.2 +/- 0.6% for arachidonic acid. The proportion of linoleic acid oxidized to CO2 did not correlate with the estimated conversion to long-chain polyunsaturated metabolites. Breast-fed newborn infants synthesize n-6 long-chain polyunsaturated fatty acids already during the first week of life, but the contribution of endogenous synthesis to the total plasma long-chain polyunsaturated pool is small. A major portion of dihomo-gamma-linolenic acid is converted to arachidonic acid.