A new noninvasive method for scintigraphic demonstration of the anatomic localization and function of the thoracic duct is described. Seven volunteers ingested 123I-heptadecanoic acid; approximately one hour later the entire thoracic duct was demonstrated and correspondingly a sharp rise in the plasma appearance rate of radioactivity was recorded. The method is simple and without side effects. The usefulness of this examination in clinical practice remains to be elucidated.