Arginases compete with nitric oxide (NO) synthases for L-arginine as common substrate. Pulmonary vascular and airway diseases in which arginase activity is increased are associated with decreased NO production and reduced smooth muscle relaxation. The developmental patterns of arginase activity and type I and II isoforms expression in the lung have not been previously evaluated. Hypothesizing that lung arginase activity is developmentally regulated and highest in the fetus, we measured the expression of both arginase isoforms and total arginase activity in fetal, newborn and adult rat lung, pulmonary artery, and bronchial tissue. In addition, intrapulmonary arterial muscle force generation was evaluated in the absence and presence of the arginase inhibitor nor-NOHA. Results: Arginase II content, as well as total arginase activity was highest in fetal rat lung, bronchi and pulmonary arterial tissue, decreased with age (P<0.05) and its lung cell expression was developmentally regulated. In the presence of nor-NOHA, pulmonary arterial force generation was significantly reduced in the fetus and newborn (P<0.01), but no significant change in force generation in bronchial tissue was noted following arginase inhibition. In conclusion, arginase II is regulated developmentally and both expression and activity are maximal during fetal life. We speculate that the maintenance of a high pulmonary vascular resistance and decreased lung nitric oxide production prenatally may in part be dependent on increased arginase expression and/or activity.