Development in chronic hypoxia has been shown to have a significant negative impact on the developing cardiovascular system. The developing chicken embryo has two ductus arteriosi (DA) that shunt blood away from the lungs to the systemic circuit and constrict during hatching in response to an increase in arterial partial pressure of O(2). The goal of this study was to determine the influence of O(2) levels during incubation on the vascular reactivity of the O(2)-sensitive DA using the chicken as a novel model system. In addition, we measured blood gas and air cell O(2) during these developmental stages. Chicken embryos were incubated in hypoxia (15% O(2)), normoxia (21% O(2)) or hyperoxia (30% O(2)) and examined on incubation days 16 and 18 and as internally pipped and externally pipped embryos. The vasoreactivity of the DA was measured in response to an increase in O(2) and during stepwise increases in noradrenaline (NA) and phenylephrine (PE). The DA from embryos incubated in hypoxic conditions contracted in response to O(2) at a later hatching stage than the DA from embryos incubated in normoxic or hyperoxic conditions. The DA from day 18 embryos incubated in hypoxic conditions had a significantly weaker contractile response to NA and PE when compared with the DA from embryos incubated in normoxic or hyperoxic conditions. Blood gas and air cell O(2) were lowest for embryos incubated in hypoxic conditions and highest for embryos incubated in hyperoxic conditions. Incubation in hypoxia significantly delays the maturation of DA, while incubation in hyperoxia accelerates development.