AIM:To explore whether the balance between endothelin(ET) and nitric oxide(NO) plays an important role in airway hyperresponsiveness of asthmatic rats. METHODS:The tension of isolated perfused rat tracheal rings was measured after ET-1 stimulation and incubation of JKC 302 and L-NAME. RESULTS:ET-1 constricted isolated rat tracheal ring, produced slowly developing and long-lasting contraction. The ET-1-induced contraction response of asthmatic rat tracheal ring was higher than that of normal control group (P＜0.01). JKC 302, a selective ETA receptor antagonist, partly blocked ET-1-induced contraction in asthmatic rat trachea ring. L-NAME significantly augmented the constriction caused by ET-1. CONCLUSION:The effects of ET on bronchomotor tone may be modified by NO as this is a bronchomotor, and the imbalance between ET and NO may play an important role in asthma pathogenesis.