The development of the research on xenobiotics has made it necessary to improve our knowledge of the action of such compounds on living systems. While the pharmacological or toxicological action is rather well understood, it is very important to have the most precise data about the metabolism of such compounds in animal bodies, in order to devise experiments which will help to collect data about metabolism in humans.

The field of metabolic studies has been evolving in the last 30 years. However, among all technical progresses, the use of molecules labeled with radioisotopes has certainly been one critical step in the development of metabolic studies of foreign compounds. While radioactivity counting using mainly scintillation techniques is still very useful for providing quantitative data about absorption, amount of unchanged products, and metabolites fixed in tissues, excretion rates, etc., this only gives a very general idea about the turnover of the compound in the body. In most cases, by using such techniques, it is very difficult to obtain precise information about the fate of xenobiotics in very small organs such as glands, fetal organs, or brain nuclei. However, such information is very important for toxicologists who want to obtain precise data in this field in order to perform the most exacting and complete toxicological studies. This is the main interest of autoradiographic techniques which help to follow labeled compounds using the photographic effect of isotopes. First introduced in 1954 by Sven Ullberg, whole body autoradiography has been largely developed in the field of metabolic studies,'where it is used to study the distribution of labeled compounds and their metabolites in animal tissues and body fluids. Since then, it has been much improved, mainly in Ullberg's department, so that this technique is now very powerful. The reader will find information in several reviews which have already been published. 2-'4 In this paper we shall first summarize technical aspects. Then in a second part we shall discuss the usefulness of this technique in obtaining kinetic data. In a third and final part, toxicological implications of whole-body autoradiographic data will be presented.