Twenty-nine premature infants were studied to determine whether neonatal asphyxia, apnea, and low blood pressure in the first day of life are associated with elevated plasma β-endorphin concentrations. Plasma β-endorphin levels Were determined at 0.5 to 2,4 to 6, and 18 to 24 hours of life, using radioimmunoassay. Premature infants with moderate or severe asphyxia (n=19) had higher levels at 0.5 to 2 hours of age (32.1 ±6.7 vs 16.4±7.4 pmol/L) and significantly higher levels at 4 to 6 hours of age (50.4±10.0 vs 22.9±9.2 pmol/L) compared with the ten nonasphyxiated premature infants. A significant elevation in levels at age 0.5 to 2 hours (39.4±9.9 vs 17.7±4.4 pmol/L) and age 4 to 6 hours (59.3 ±13.8 vs 27.1 ±17.1 pmol/L) was observed in premature infants with low blood pressure or impaired perfusion (n=12) who required the administration of volume expanders. No differences Were observed in premature infants with anwithout apnea. It may be speculated that the increased endogenous release of β-endorphins in response to perinatal asphyxia may play a role in the pathogenesis of shock observed in the first day of life.