To measure the disappearance rates and distribution of luteinizing hormone-releasing hormone (LH-RH), 125I-monoiodo-LH-RH was produced by the lactoperoxidase technique and separated from noniodinated LH-RH and two oligomers of iodinated LH-RH by polyacrylamide gel electrophoresis. 125I-LH-RH was injected iv into three normal men. Least-squares fitting by computer established that a three-term exponential equation was necessary and sufficient to describe the disappearance curves. The t½ of each component was: 2–4 min, 30–60 min, and 600–1300 min. The initial distribution space approximated plasma volume. Similar studies in the rat yielded disappearance curves best characterized by two exponentials ranging from 5 to 10 min for the first component and 150 to 600 min for the second. The initial distribution volume in the rat also approximated estimated plasma volume and 125I was concentrated in the pituitary gland and kidney. The disappearance curves and tissue distribution of two oligomers of iodinated LH-RH in the rat were different from those of 125I-LH-RH.