Lipolysis has been measured in humans by means of stable isotope techniques using labeled palmitic acid (PA) or glycerol as tracers. If other fatty acids (FA) such as linoleic acid (LLA) have the same rate of appearance (Ra) as PA and therefore contribute equally to oxidative and nonoxidative metabolism is unknown. We infused albumin-bound [U-13C]PA and [U-13C]LLA in seven critically ill infants (weight 3.6 +/- 1.3 kg, age 57 +/- 64 d) receiving 20.9 +/- 5.4 kcal. kg-1.d-1 of i.v. glucose only, and measured simultaneously the Ra of PA and LLA from the isotopic enrichment of plasma FFA by mass spectrometry. A needle biopsy of the s.c. adipose tissue was obtained for FA composition. PA in adipose tissue was higher than LLA (40 +/- 6.7 versus 5.4 +/- 3.2 mol %, p < 0.001). The Ra values of PA and LLA were 5.73 +/- 2.79 and 1.34 +/- 0.92 mumol.kg-1.min-1, respectively (p = 0.005). However, the ratio of the FA's Ra to their respective mol% values in adipose tissue was lower for PA than for LLA (0.15 +/- 0.06 versus 0.25 +/- 0.06, p = 0.02). The Ra of LLA acid was higher than could be expected from the FA composition of adipose tissue, thus indicating a preferential release of LLA during lipolysis. In critically ill infants receiving only i.v. glucose, the contribution of LLA to the oxidative and nonoxidative metabolism may be larger than what assumed from the FA composition of plasma and adipose tissue.