The relationship between inositol phosphates metabolism and inhibition of DNA synthesis was examined. The DNA synthesis rate measured by the ³H-labeled thymidine incorporation into cellular DNA in NIH3T3 fibroblast treated with UV indicates that there exists a resting stage of DNA synthesis in cells exposed to UV light. Intracellular levels of inositol phosphates in cells were decreased within 5 minutes after UV treatment. The size of inositol-1,4,5-triphosphate pool in NIH3T3 cells was most sensitive to UV treatment. Competitive binding assay was carried out for the measurement of intracellular IP₃ contents based on the competition for the binding sites with lable and unIalled IP₃. Microsomal preparations from bovine adrenal cortex were used as binding protein. This study indicated that UV irradiation might indirectly affect DNA synthesis by impediment of inositol phosphate metabolism in the cells.