The mean plasma ammonia level at birth of 36 very low birth weight infants (< or = 32 weeks of gestation) was 71 +/- 26 mumol/L (121 +/- 45 micrograms/dl), which is similar to the mean level in preterm infants born at > or = 32 weeks of gestational age. Plasma ammonia levels declined to 42 +/- 14 mumol/L (72 +/- 24 micrograms/dl) at 7 days of age; mean ammonia levels at 14, 21, and 28 days of age were similar to that at 7 days of age and to the mean plasma ammonia level of 14 healthy term infants at birth (45 +/- 9 mumol/L (77 +/- 16 micrograms/dl)).