The dynamic of the whole body retention in rats after iv ,75,Semethionine was determined by whole body counting. The retantion was well described by a three-component equation: R (t)=24.06 e,-0.679t,+41.24 e,-0.042t,+15.41 e,-0.005t, wherein the t,1/2, were T,1,=1.0 d, T,2,=16.4d,T,3,=132 d. The radioactivities in urine revealed 2 components with t,1/2,=0.6 and 9.8 d respectively. Biological t,1/2,=5.2 d in fecal excretion for the fast component and 13.2 d for the long term component. The autoradiographic study indicated that after iv ,75,Se-methionine in rats the radioactivity accumulated chiefly in pancreas, and then in liver, skeleton and thyroid gland, only a trace was found in hair. A method was studied for rapid preparation of autoradiographs for tissue slices by impregnation of nuclear emulsion with liquid scintillators PPO and POPOP. Mixing liquid scintillators, especially a hydrophilic dioxane solvent, with emulsion allowed also rapid autoradiographic grain production. The tissue distribution of ,75,Se-methionine by fluorescence-sensitized autoradiography was observed after iv ,75,Se-methionine. The number of autoradiographic grains/10 µm,2, was increased with the scintillator by 4—10 times. The silver grains in the autoradiograms were distributed high in the pancreatic acini and the islets, and then in liver cells, bone marrow and epithelial cells of thyroid gland.