Objective To investigate changes in endothelin 1 (ET 1) levels in neonates with hypoxic ischemic encephalopathy (HIE). Methods Serum and CSF ET 1 levels in 20 normal neonates and 52 HIE neonates were assayed using radioimmunoassay and the 52 HIE neonates were divided into mild, moderate, and severe groups. Results Serum ET 1 levels were 68.71±12.03 ng/L in the normal control group. In the aucte phase, serum ET 1 levels in infants with mild, moderate, and severe HIE were 98.38±12.82 ng/L, 107.11±18.56 ng/L, 119.56±14.69 ng/L, respectively; and in the remission stage they were 73.44±11.79 ng/L, 75.73±11.38 ng/L, 83.92±15.99 ng/L, respectively. Serum ET 1 levels were significantly increased in the acute phase compared to the remission stage and the controls (P<0.01). In the acute phase, CSF levels in infants with mild, moderated, and severe HIE were 43.79± 7.14 ng/L , 51.07±11.19 ng/L, 61.86±13.55 ng/L, respectively; and in the remission stage they were 30.79± 4.42 ng/L , 33.07±4.84 ng/L, 39.50±5.56 ng/L, respectively. In the acute phase, CSF ET 1 levels in infants with moderate and severe HIE were much higher than those in the controls (P<0.01 and P<0.05), although in the recovery phase, the ET 1 levels of the mild and moderate HIE groups decreased to normal. ET 1 levels in severe HIE infants remained at a high level. Serum and CSF ET 1 levels were significantly correlated with the severity of HIE and were negatively correlated with 1 minute Apgar score. Conclusions ET 1 may play an important role in the pathogenesis of HIE.