The rate of glucose turnover (R_a) and gluconeogenesis (GNG) via pyruvate were quantified in seven full-term healthy babies between 24 and 48 h after birth and in twelve low-birth-weight infants on days 3 and 4 by use of [¹³C₆]glucose and ²H₂O. The preterm babies were receiving parenteral alimentation of either glucose or glucose plus amino acid with or without lipids. The contribution of GNG to glucose production was measured by the appearance of ²H on C-6 of glucose. Glucose R_a in full-term babies was 30 ± 1.7 (SD) µmol · kg¹ · min¹. GNG via pyruvate contributed ~31% to glucose R_a. In preterm babies, the contribution of GNG to endogenous glucose R_a was variable (range 6-60%). The highest contribution was in infants receiving low rates of exogenous glucose infusion. In an additional group of infants of normal and diabetic mothers, lactate turnover and its incorporation into glucose were measured within 4-24 h of birth by use of [¹³C₃]lactate tracer. The rate of lactate turnover was 38 µmol · kg¹ · min¹, and lactate C, not corrected for loss of tracer in the tricarboxylic acid cycle, contributed ~18% to glucose C. Lactate and glucose kinetics were similar in infants that were small for their gestational age and in normal infants or infants of diabetic mothers. These data show that gluconeogenesis is evident soon after birth in the newborn infant and that, even after a brief fast (5 h), GNG via pyruvate makes a significant contribution to glucose production in healthy full-term infants. These data may have important implications for the nutritional support of the healthy and sick newborn infant.