The oxygen tensions in the spleen of mice forced to breathe 10%, 7%, and 4.6% oxygen were, respectively, 0.36, 0.27, and 0.10 of normal. The dose reduction factors at the corresponding levels were 1.24, 1.73, and 1.96. Serotonin reduced the oxygen tension to 0.53 of normal and gave a dose reduction factor of 1.77. PAPP reduced the oxygen tension to 0.35 of normal and gave a dose reduction factor of 1.74. A combination of serotonin and an atmosphere of 4.6% oxygen, where the oxygen tension in the vena cava declined to 0.06 of normal, gave a dose reduction factor of 2.92. If one plots the relative effectiveness of the radiation (y = 1/DRF) versus the relative oxygen tension (x) of the spleen and vena cava for each of the experimental situations, there exists an approximate linear relationship for values of x not greater than 1. The results indicate that there is a small amount of protection by both serotonin and PAPP which is independent of hypoxia. This relationship can be given as <tex-math>$F_{i}y$</tex-math> = 0.42 + 0.60x, in which the values of Fi are 1.4, 1.2, and 1.12, where i refers, respectively, to serotonin, sodium nitrite, and PAPP.

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