OBJECTIVE: Xinmailong, a compound extracted from periplaneta americana, is used for the treatment of cardiovascular diseases. This study investigated the effects of Xinmailong on myocardial hypoxia-inducible factor-1α (HIF-1α) and plasma endothelin-1(ET-1) levels in neonatal rats with asphyxia and explored the protection mechanism of Xinmailong in hypoxia-ischemic myocardial injury. METHODS: Seven-day-old Sprague-Dawley rats were randomly divided into three groups (n=30 each): sham-operated, asphyxia, Xinmailong-treated asphyxia. Each group was randomly subdivided into three groups according to the observed time points: 6 hrs, 24 hrs and 72 hrs. Xinmailong (5 mg/kg) was intraperitoneally injected to the rats in the Xinmailong-treated group five minutes before asphyxia. Myocardial HIF-1α expression, and plasma ET-1 and creatine kinase (CK) levels were measured. The histopathologic changes of the myocardium were observed by hematoxylin-eosin staining. RESULTS: Four rats died in the asphyxia group while only one died in the Xinmailong-treated group during the experiment. The plasma ET-1 and CK levels as well as myocardial HIF-1α expression increased at 6 hrs, reached a peak at 24 hrs, and declined at 72 hrs after asphyxia in the asphyxia group, being higher than that in the sham-operated group (P<0.01). Myocardial ischemia was observed in the three time points, and cell necrosis occurred at 24 hrs after asphyxia in the asphyxia group. Myocardial HIF-1α expression was positively correlated with plasma ET-1 levels (r=0.876, P<0.01). In the Xinmailong-treated group, plasma levels of CK and ET-1 as well as myocardial HIF-1α expression were significantly lower than those in the asphyxia group (P<0.01). Myocardial ischemia was alleviated and no cell necrosis was found in the Xinmailong-treated group. CONCLUSIONS: Asphyxia leads to increase in myocardial HIF-1α expression and plasma levels of ET-1 and CK. Xinmailong can reduce the myocardial expression of HIF-1α and decrease plasma ET-1 levels, thus alleviating hypoxia-ischemic myocardial injury.［Chin J Contemp Pediatr, 2009, 11 (8):683-686］