The distribution of i.v. injected S-adenosyl-L-(methyl-14C) methionine was studied in the whole body of mice and in the cat brain using an autoradiographic technique. The drug was concentrated, at shorter survival times, mainly in the blood stream and in highly vascularized organs. Low concentration of radioactivity was found at longer times in the central nervous system (CNS). The quantitative analysis by means of organ radioassay was carried out in rats and confirmed the autoradiographic data.