The single circulation myocardial extraction of terminally iodinated hexadecenoic acid (77 plus or minus 11.0%) is approximately two times the extraction of 18 carbon fatty acids (33 plus or minus 5.2%) prepared by iodination of double bonds. The results compare favorably with natural 28 carbon steraic (70 plus or minus 7.5%) and oleic (61 plus or minus 7.8%) acids labeled with carbon-11 in the carboxyl group. It is concluded that terminally radioiodinated long chain fatty acids can be used as substitues for in vivo investigations of fatty acid distribution and in particular may be useful as a regional mycardial blood flow indicator.