Elimination of human urinary trypsin inhibitor (UTI) after intravenous injection of ^l25I-labelled UTI was followed by serial plasma and urine samples in three male volunteers. The plasma half-life of ^l23IUTI during 0-3 h after injection was 33 min and during the following 4 hours the half-life was 2 hours. Free, biologically active inhibitor was found in the urine during the first four hours after injection. Most of the radioactivity in the urine, however, corresponded to free ^l25I probably released during the degradation of UTI in the kidney. The distribution of UTI was studied after injection of ^l25IUTI in rats by measurement of radioactivity in excised organs. Fifteen min after the injection 44% of the radioactivity was found in the kidneys and 9% in the liver, implying that the kidneys are the primary site of UTI metabolism.