It has been suggested previously that delayed closure of the ductus arteriosus in premature infants is related to an ineffective constriction in response to an increase in arterial PO2. The contractile effects of increased PO2and excess K+were studied in rings of ductus arteriosus from early (70 ± 4 days, n> = 9) and late (137 ± 3 days, n> = 11) gestation fetal lambs. Studies were performed in a laboratory using overhead fluorescent lighting or in a dark, enclosed box. Room light relaxed the oxygen-induced contraction in immature vessels but had no significant effect on the K+-induced contraction. Room light did not alter either the oxygen or K+-induced contractile responses in mature vessels. When comparing oxygen induced contractions in room light in immature vessels (0.27 ± 0.13 g, n> =9) vs. mature vessels (0.82 ± 0.06 g, n> = 11) there appeared to be an increased response to oxygen with advancing gestational age. However, when the oxygen-induced responses of immature (0.59 ± 0.15 g, n> = 9) and mature (0.82 ± 0.06 g, n> = 11) vessels were performed in an environment excluding room light, no significant gestational difference was observed. The role of oxygen in delayed closure of the ductus arteriosus of premature infants will need further evaluation.SpeculationThe difference in sensitivity to photorelaxation between rings of ductus arteriosus from immature and mature lambs may be associated with biochemical differences in vessels between early and late gestation.