The nuclear age, which has been with us slightly more than 20 years, has brought with it an unusual awareness of a relatively new toxic agent--ionizing radiation. In fact, a new science, health physics, was created to give special attention to this problem. As a consequence and in spite of the unparalleled hazards associated with ionizing radiation, this new nuclear industry is growing rapidly into a benevolent giant bringing a better way of life while at the Same time maintaining radiation damage at an insignificant level. Although i n the past few decades we have learned much more about the hazards associated with ionizing radiation than those associated with some of the common industrial hazards and although maximum permissible exposure levels for the radionuclides have been established with greater reliability and confidence than have the levels for many chemical agents with which man has been familiar for many centuries, there still remains a considerable uncertainty in many of the basic assumptions and in the parameters used in the calculation of maximum permissible body burden and maximum permissible concentration of the various radionuclides in food, water and air. There is need to determine the uptake, distribution and elimination of a variety of chemical compounds of the approximately 300 common radionuclides. These data are needed for the several modes of intake by the various age groups, and differences due to race, sex, weight, eating habits, etc., should be investigated. There is need especially to obtain data from studies of human exposure and to examine the influence of the quantity and chemical form of the radionuclide and of other associated chemical elements taken into the body, both from single exposure and from continuous exposure.