Kinetics of glycerol metabolism and triglyceride/fatty acid cycling were quantified in 12 healthy, normal, appropriate-for-gestational-age (AGA) infants, eight small-for-gestational-age (SGA) infants, and five infants of insulin-dependent diabetic mothers (IDM) at less than 48 h of age. Stable isotope-labeled [2-13C]glycerol and [6,6-2H2]glucose in combination with indirect respiratory calorimetry were used. The tracers were used as constant rate infusion and steady state isotopic enrichment of glucose, glycerol, and bicarbonate was measured by mass spectrometric methods. After a 7− to 9-h fast, the plasma glucose, glycerol, and FFA concentrations were similar in the AGA and IDM groups. In the SGA group, the plasma glucose concentration was significantly lower than that in the AGA group throughout the study, but plasma FFA and glycerol concentrations were not different from those in the AGA infants. Plasma betahydroxybutyrate concentration was significantly elevated in the AGA group compared with IDM and SGA infants (AGA 0.59 ± 0.39, SGA 0.35 ± 0.09, IDM 0.33 ± 0.21 mmol/L; mean ± SD). The rate of appearance of glycerol was significantly elevated (p< 0.05) in SGA infants (AGA 9.47 ± 2.11, IDM 9.55 ± 2.14, SGA 12.15 ± 3.87 μmol/kg·min). Between 80 and 90% of glycerol turnover was converted to glucose, accounting for 20% of glucose turnover with no significant difference in the three groups. Approximately 35% of glycerol carbon was recovered in the bicarbonate (CO2) pool. Less than 5% of CO2carbon was derived from glycerol. Estimation of triglyceride-fatty acid cycle revealed that the triglyceride energy mobilized was increased in SGA infants. Only 22–24% of the triglyceride energy released was oxidized to CO2in the newborn infants; the majority (76–78%) was recycled back to the adipose tissue. These data show that lipolysis is active in the immediate neonatal period. The contribution of fat to oxidative metabolism is increased in SGA infants. The major metabolic fate of glycerol in the neonate is conversion to glucose, and glycerol is a minor contributor to oxidative metabolism. (Pediatr Res31: 52–58, 1992)