We hypothesized that high cord blood Ep levels measured by radioimmunoassay, reflecting in utero hypoxic stress in some apparently normal term infants, might be associated with increases in the pulmonary excretion rate of CO (VeCO), an index of bilirubin production, because of consequent effective erythropoiesis or stimulation of hepatic heme turnover by other biochemical effects of such stress. Forty-eight normal-sized (AGA) or macrosomic (LGA; > 4 kg) infants were studied consecutively, excluding only those with a diabetic mother (IDMs; maternal HbAIc > 5.7% total Hb), a hematoma, bruising, or a positive Coombs test. Others with a history of possible in utero hypoxic stress late in gestation (toxemia, 2nd stage of labor > 2 hrs, dystocia, meconium staining, nuchal cord, low Apgars) were included (9/26 AGA; 14/22 LGA). Ep and VeCO did not correlate. No infants had polycythemia. Thus, if IDMs are excluded, 1) AGA and LGA infants with nonchronic in utero hypoxic stress late in gestation may have similar postnatal bilirubin production, and 2) cord blood Ep levels in such infants may not predict bilirubin production after birth.