Mode of action study of irradiation was performed with potato tuber slices, $1mm{ imes}1.5cm,$ aged on moist filter paper under aseptic technique. The time courses of invertase activity, nucleic acids and respiratory activity were determined, and sensitivities of these three processes to ionizing radiation were measured. None of those processes was severely inhibited by the dosage suppressing cell division. The result of $^3H-thymidine$ incorporation suggests that the reaction site of ionizing radiation might be existent during mitosis or $G_2$ period.