Magnesium causes a variety of vascular smooth muscle to relax. The present study was designed to determine whether there is a developmental change in the magnesium-induced response of pulmonary vasculature. Isolated pulmonary arteries (PA) of newborn (1- to 3-day-old) and juvenile (4- to 6-wk-old) rabbits were suspended in organ chambers filled with modified Krebs-Ringer bicarbonate solution (95% O₂-5% CO₂, 37.0°C), and their isometric tension was recorded. In arteries preconstricted with endothelin-1 to a similar tension level, MgSO₄ caused greater relaxation of juvenile rabbit PA than that of the newborn rabbit PA. Verapamil, a voltage-dependent Ca²⁺ channel blocker, attenuated magnesium-induced relaxation in juvenile rabbit PA but not in newborn PA. The uptake of Ca²⁺ of juvenile rabbit PA was inhibited by MgSO₄, and the inhibition was attenuated by verapamil. The uptake of Ca²⁺ of newborn rabbit PA was smaller than that of the juvenile PA and was not significantly affected by MgSO₄ and verapamil. These results demonstrate that there is a developmental increase in the dilator effect of MgSO₄ in rabbit PA. In newborn rabbit PA, an incomplete maturation of the voltage-dependent Ca²⁺ channels may contribute to the smaller vasodilation induced by MgSO₄.