Maternal venous (MV) and neonatal umbilical venous (UV) and umbilical arterial (UA) plasma beta-endorphin concentrations were measured at birth in two groups of patients undergoing cesarean section (CS). Patients in group 1 carried a term gestation (n = 12, birth weight 3,640 +/- 150 g, 1 SEM). Patients in group 2 (n = 12) carried a preterm gestation (birth weight 1,956 +/- 150 g). In the term neonate the UV and UA plasma beta-endorphin were 11.5 +/- 4 and 13.5 +/- 4 picomoles/liter, respectively. In the preterm neonate the UV and UA values were 23.6 +/- 4 and 23.5 +/- 5 picomoles/liter, respectively. Both values in the preterm neonate were significantly greater than the corresponding value in the term neonate (p less than 0.05). The MV plasma beta-endorphin did not significantly differ among the two groups. Data suggest that in preterm neonates increased plasma beta-endorphin levels occur at the time of birth.