Endothelin-1 is involved in the pathogenesis of respiratory distress in neonates. So, the plasma level of Endothelin-1 was measured in preterm infants with respiratory distress syndrome. It was also measured in neonateswith meconium aspiration syndrome and transient tachypnea of the newborn.In addition plasma ET-1 was also assayed in 15 normal preterm and 15normal fullterm infants as a control group.ET-1 plasma levels were assayed on the second day of life usingenzyme immmmunoassay technique.Plasma ET-1 concentrations were significantly higher in patients withRDS group than other groups. There was no significant statistical differencebetween males and females included in the study as regards plasma level ofET-1. Also, there was no significant correlation between infants delivered bynormal vaginal delivery and those delivered by caesarean section as regardsmean plasma level of ET-1.The present study found that ET-1 levels in the infants who died fromrespiratory distress were significantly higher than the survivors.A negative correlation between plasma ET-1 levels and each of birthweight, gestational age, Apgar scores at 1 and 5 minutes was noted. Alsothere was moderate positive correlation between plasma ET-1 levels and FiO2(and hence respiratory distress severity) required in cases that neededventilatory support either nasal CPAP, or SIMV.Also there was weak negative correlation between plasma ET-1 levelsand PH, PO2, HCO3, BE and O2 saturation. A weak positive correlationbetween plasma ET-1 levels and PCO2 was observed. There was weakpositive correlation between plasma ET-1 levels and Downes' score.Our study concluded that plasma level of endothelin-1 can be used indifferentiating cases of RDS from other common causes of respiratory distressin neonates.Higher plasma ET-1 levels are indicative of the severity of neonatalrespiratory distress in the early course of the disease and can be used as apredictor of poor outcome