【Objective】The mechanisms of the O₂-induced ductal constriction still remain unclear. Membrane potentials of the vascular smooth muscle cells decrease with the elevation of PO₂ after birth, which are mainly determined by potassium channels on the membranes, including: BK_Ca, K_ATP, K_V and K_ir channels. Among them there was no report on BK_Ca channels both α and β subunits in ductus arteriosus (DA). Our purpose was to investigate the developmental changes of BK_Ca in DA.【Method】BK_Ca channels have 5 subunits: α, β₁, β₂, β₃, β₄. Their expression in mRNA level were examed in the tissues from 19th and 21th days rat fetuses as well as 87th days porcine fetus and newborn, separately, including DA, aorta (AO), pulmonary artery (PA), heart, lung and brain by Real-time PCR.【Results】BK_Ca α and β₁ subunits mRNA expressed higher in DA than in PA and AO in porcine, moreover, their quantity increased along with the development. While as to rat fetuses, the expression of b1 subunits were much higher than α in DA, AO and PA. Along with the development, the expression of β₁ subunits increased in DA and AO, but decreased in PA. The expression of α and other β subunits were quite low in the vasculars in both rat fetuses.【Conclusion】BK_Ca β₁ subunits may play an important role in the after birth adaptation of the DA and PA in mammals.