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Conventional whole-body autoradiography (MAR) shows detailed tissue distribution of radioisotopes “relatively”, not “quantitative”. We attempted to establish quantitative analytical method for MAR (QMAR) using computer-assisted densitometry system introduced from Dr. Sokoloff's laboratories, N.I.H. and Imaging-plate system developed by Fuji Photo Film Co., Ltd. Autoradiograms were prepared from rats after administration of (CH3-14C) FO-1561 (S-adenosylmethionine sulfate tosylate, SAMe) and exposed on X-ray films (Kodak, SB-5) for 3.5 days or Imaging-plates for 1.5 hrs with 14C-staircases. Measurement of isotope concentration was done from the autoradiograms by comparing optical densities or detection intensities of the regions of interest with a series of standards. Concentrations of SAMe-14C in blood, muscle, thymus, submaxillary gland and brain obtained from QMAR methods were comparable to those obtained from conventional tissue dissection method (LSC). Correlation coefficient between QMAR and LSC method was lager than 0.9. In addition, quantitative information was obtained on the distribution in such small organs which can not be dissected or on the localization of radioactivity in organ or tissue.