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Segments of human umbilical arteries and rings from the ductus arteriosus of lambs, rabbits, and monkeys were studied in a constant-temperature bath of Tyrode's solution. Gas mixtures of high oxygen tension produced contraction in 70% of the umbilical arteries, 90% of the lamb ductus, 67% of the monkey ductus, and only 28% of the rabbit ductus. Failure to contract was not related to the interval between delivery and study. Since no appreciable plasma or granulocytes were present in the specimens, it appeared that bradykinin was not responsible for the reversible contraction with increased [Formula: see text].

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