Extract: Development of the constrictor response of the ductus arteriosus to O 2 and selected vasoactive drugs (acetylcholine and bradykinin) was studied in 20 fetal lambs weighing 680–4800 g (90− to 150-day gestation). The isolated ductus arteriosus was perfused in vitro with Tyrode's solution at constant flow, temperature (38), pH (7.3–7.4), and P CO 2 (30–40 mm Hg), and the mean pressure difference across the ductus was measured. Ductal resistance was calculated at different levels of P O 2 (10–700 mm Hg), raised stepwise to produce dose-response curves. Three young fetuses failed to respond initially to O 2 and in the other 17 the initial response occurred at progressively lower P O 2 levels with advancing gestation. The maximal degree of constriction developed showed a progressive increase with advancing gestational age.