Hexadecenoic acid, terminally labeled with 123I, provides a compound that concentrates in the myocardium in proportion to relative regional blood flow and carries a .gamma.-emitter with desirable detection and imaging qualities. After i.v. injection in experimental animals, the clearance half-times of hexadecenoic acid for blood and myocardium are 1.7 and 20 min, respectively. These values compare favorably with 18-C fatty-acid analogs labeled with 11C. In acute and chronic infarction, similar distribution patterns are found for hexadecenoic acid and 43K, which indicates that hexadecenoic acid is a suitable substitute for the K analogs now in use for myocardial imaging. Because of the high count rates obtainable with 123I-hexadecenoic acid, good-quality images can be acquired in as little as 2-3 min per view. Iodine-123-hexadecenoic acid is potentially a useful radiopharmaceutical for clinical application.