In view of the potentiation of the biologic effects of ionizing radiation by increased oxygen tension in tissues, the relationship of oxygen tension to the therapeutic and toxic actions of alkylating agents has been examined. Changes in oxygen concentration at 1 atmosphere of pressure from 8 to 100 percent did not alter the toxicity of nitrogen mustard to ICR/Ha mice. Similarly, no changes were seen when 100 percent oxygen was given at 2 to 5 atmospheres of pressure. Increase in oxygen tension in tissues was demonstrated by continuous polarography. This increase did not alter the effect of nitrogen mustard on increasing survival time or decreasing tumor growth of mice inoculated with Ehrlich adenocarcinoma, Sarcoma 180, and Leukemia L1210 in either the ascites or the solid form. Exposure of ascites tumors to nitrogen mustard in vitro produced equal prolongation of survival time of the recipient mice whether exposure was undertaken under normal atmospheric conditions or increased oxygen tension. With paramagnetic resonance spectroscopy, no free radicals were detected in frozen solutions of nitrogen mustard, chlorambucil, triethylenemelamine, busulfan, and triethylenethiophosphoramide. Mechanisms of the biologic effects of alkylating agents as compared to those of ionizing radiation are discussed.