The biochemical effects of ionizing radiation are many and have been reviewed recently by Ord and Stocken (11). The inhibited incorporation of radio active precursors into the deoxyribonucleic acid (DNA) of tissues of irradiated animals, supposedly representing a decrease in nucleic acid synthesis, is a biochemical effect of radiation which has been observed by many workers and which provides a plausible explanation for the pathological events which occur. However, it is not known whether the reduced incorporation of precursors represents a direct inhibition of the DNA synthetic process per se or is, instead, a reflection of a reduced avail ability of the labeled precursor due to changes in distribution, permeability, or metabolism, or to alterations in the size of pertinent metabolic pools. Further, if radiation does inhibit the synthesis of DNA, it is not known whether the inhibition is a primary effect on an enzymatic reaction or a sec ondary effect resulting from changes in other bio chemical processes or alterations in cellular ac tivity, such as mitosis. If irradiation affects the synthesis of DNA pri marily and/or directly, an inhibition might be ex pected to occur under in vitro conditions as well as in vivo. However, the incorporation of labeled pre cursors into DNA of many tissues has been found to be more radiosensitive when studied in vivothan when studied in vitro (8, 12, 13, 15). The ascites tumor, a free suspension of cells which can be in vestigated under relatively comparable condi tions in vivo and in vitro, is especially well suited to the study of factors which influence the charac ter and extent of DNA synthesis. Previously, Forssberg and Klein (1) reported that in vivo in corporation of glycine-2-C14 into the DNA of Ehr lich ascites tumor cells was reduced to 25 per cent of the control value 35-150 minutes following a to tal-body irradiation of 1,250 r, while Kelly (6) has shown that the P32incorporation into the DNA of these cells is markedly reduced 24 hours after 800 r. The present report deals with the in vivo in-Receivedfor publicationAugust10,1936. corporation of P32by four different types of ascites tumors in a 3-hour interval immediately follow ing 1,000-5,000 r total-body x-radiation. Subse quent papers concern a comparison of in vivo and in vitro radiation effects (4) and a study of other factors which influence the in vitro incorporation of nucleic acid precursors (5). MATERIALS AND METHODS Four types of ascites tumors' were studied : TAS mammary adenocarcinoma, Lymphosarcoma 6C3HED, and the tetra-ploid and hyperdiploid Ehrlich carcinomas. The latter two tumors were maintained in Carworth Farms CFi mice, where as CAFi and C3H mice from the Jackson Memorial Lab oratory were used for the TA3 and 6C3HED tumors, re spectively. Mice were given inoculations of 2 X IO7 tumor cells and used 7 days later for experiments. Several identical experiments were carried out with each tumor. For a typical experiment, 28 tumor-bearing mice were divided into four groups, one of which served as a co...