Many studies on nitric oxide (NO) as a messenger for biosignal transduction have recently been published. Endogenous NO is thought to play a pivotal role in homeostasis of the respiratory system and pulmonary circulation. As exhaled NO levels increase in patients with asthma or bronchiectasia, NO seems to be related to some pathological states in the respiratory organ. NO inhalation has been shown to be effective in attenuating pulmonary hypertension and desaturation in acute respiratory failure. Recent findings concerning NO in the lung were described in this review article.