At birth changes occur in the pulmonary circulation to facilitate the transition from placental support to air breathing. Nitric oxide is a potent vasodilator and immunostaining has demonstrated the presence of endothelial nitric oxide synthase (eNOS) in the pulmonary arteries at birth with a transient increase in the first three days of life. (Hislop et al., 1995; Arch. Dis. Child Fetal Neonatal Ed.; 73, F17-F21). However whether this NOS is active at different stages before and after birth have not been determined. Here we show that NOS activity (measured in homogenates by the conversion of 3H L-arginine to L-citrulline) is very low in porcine fetal (1 week pre-term) lung tissue. By contrast NOS activity at 5 min, 1, 3, 6, 14 day and 1-2 years after birth is high (n=4-9; Figure), with peak activity present at the 14 days of age. At each age, NOS activity was found to be totally dependent on calcium and inhibitable by L-NG-nitro-L-arginine (1mM). These data suggest a role for the rapid expression/activation of constitutive NOS in the transition from the fetus to neonate. Western blots suggest eNOS. (Graph Presented).