To elucidate the effects of birth stress on immunoreactive endothelin-1 (irET-1) concentrations in fetal blood, we determined irET-1 levels in cord plasma in different modes of delivery associated with or without complications such as asphyxia. The irET-1 concentrations in both the umbilical artery and vein were significantly higher than those found in maternal venous blood at delivery, although there was no significant difference between preterm and full-term infants. When plasma irET-1 concentrations of healthy infants born by vaginal delivery and by cesarean section without labor were compared, the former had significantly (p less than 0.05) higher levels than the latter (15.4 +/- 4.9 pg/mL versus 11.1 +/- 3.1 pg/mL). Furthermore, umbilical venous plasma obtained from vaginally delivered infants complicated by asphyxia showed significantly (p less than 0.001) higher irET-1 levels (28.2 +/- 9.4 pg/mL) than those of nonasphyxiated infants (14.2 +/- 4.5 pg/m). These data suggest that birth stress, especially asphyxia, may contribute to the increase in fetal circulating irET-1 levels.