Twelve SGA infants were studied from 4 hours after birth (day 1), before and for 4 hours after injection of 0.5 g of fat/kg b.w. (Intralipid, IL). Eight infants were restudied after 24 hours (day 2). A positive correlation was found between initial samples of FFA and glucose on day 1 (n = 11, r 0.71, p less than 0.02) and between FFA and beta-hydroxybutyrate (n = 12, r 0.62, p less than 0.05), suggesting that FFA mobilization and oxidation in SGA infants with low blood glucose. After IL infants with low blood glucose had a more pronounced defect of intravascular lipolysis. Four infants had initial hypoglycemia (HG), with blood glucose 0.4-1.3 mmol/l, and 8 were normoglycemic (NG). In the NG group initial levels of lactate and alanine were within normal limits, and no changes occurred after IL. An early peak of glycerol was seen. In the HG group initial lactate and alanine levels were higher than in the NG group, while glycerol did not differ. After injection of IL, glucose increased at 60 and 120 min in the HG group. A close correlation was found between mean levels of lactate and alanine and a negative correlation between lactate and glucose, while no correlation was found between glycerol and glucose levels. The infants with the lowest initial glucose and the highest lactate levels had the steepest rise in glucose and the fastest decrease in lactase per unit increase in beta-hydroxybutyrate after IL. On day 2 the initial levels of lactate and alanine were lower than on day 1 and all infants were normoglycemic. A glucose peak corresponding to the glycerol peak was seen after IL, but lactate and alanine levels did not change. These data were consistent with reduced lipolytic capacity, low fatty acid oxidation and reduced gluconeogenesis in SGA infants on day 1, especially in those with HG. The glucose and beta-hydroxybutyrate levels were rapidly increasing and lactate levels decreasing after IL, suggesting improving gluconeogenesis concomitantly with increasing fatty acid oxidation.