THE recent communication of Winteringham1 has focused attention on the two main difficulties which limit the value of the autoradiographic method in the study of the distribution of soluble radioisotopes of short half-life. These two problems, that of preparing the tissue without loss of the soluble tracer, and the enormous radiation intensities needed for a successful exposure, have been largely solved in our work; and in view of the general usefulness of the methods employed, a preliminary publication of their essential features may not be out of place.