In addition to passive factors, pulmonary vascular tone is under the control of adrenergic, cholinergic, NANC vasodilator nerves and humoral mechanisms. Hypoxic pulmonary vasoconstriction plays an important role in the active control of pulmonary vascular tone. Adrenergic nerves, HPV and vasoconstrictor humoral substances are the vasoconstricting forces, whereas cholinergic, NANC mechanisms and vasodilator humoral substances are the dilating factors. Basal and stimulated release of NO from endothelial cells also represents dilating force. Thus, a balance between these two sets of opposing forces determines the normal resting pulmonary vascular tone. Disturbance of this balance may result in or/and contribute to the development of some pulmonary vascular diseases such as pulmonary hypertension. Endothelium-derived NO plays an important role in the regulation of pulmonary vascular tone. It inhibits adrenergic contraction, hypoxic pulmonary vasocon-striction and the contractile response to many vasoconstrictors. NO mediates the vasodilator responses to NANC stimulation and variety of pulmonary vasodilator substances. NO also play a part in the perinatal pulmonary vascular adaptation.