The influence of postnatal age on the distribution of vascular resistances was studied in in situ, isolated, blood perfused lungs of lambs aged 6-12 h, 2-4 d, 2 wk, and 1 mo. The pressure gradients across the upstream, downstream, and compliant middle segments were determined by an inflow-outflow occlusion technique during mild hyperoxia (FIO2 = 0.28) and severe hypoxia (FIO2 = 0.04) under control and indomethacin-treated conditions. The greatest response to hypoxia was seen in the middle segment. A lesser, but significant response was also seen in the upstream segment. Indomethacin accentuated the hypoxic response of the total circuit and the middle segment at all ages, and the upstream segment in the two younger age groups. An age-dependent increase in total hypoxic reactivity was seen in control lungs, but not in indomethacin-treated lungs. When the pressure gradient across the middle segment alone was considered, an age-dependent increase in hypoxic reactivity was seen in controls and a decrease was seen in indomethacin-treated lungs. In contrast there were no age-dependent changes in the hypoxic responses of the upstream or downstream segments. These data suggest that the predominant site of hypoxic pulmonary vasoconstriction and dilator prostaglandin activity lies within the middle gradient in newborn lamb lungs. Furthermore, the age-dependent changes in the hypoxic response of the middle gradient appear to be altered by the modulating influence of dilator prostaglandins.