[Monoiodo- Tyr3 ]neurotensin, a neurotensin analogue that contains a single iodine atom on the side chain of Tyr 3 was prepared and purified. This analogue can be labeled at any specific radioactivity between 0 and 2000 Ci/mmol; its binding and biological properties on rat and guinea pig neurotensin receptors are identical to those of the parent peptide. These properties make [monoiodo- Tyr3 ]neurotensin the best suitable radioligand for detection and characterization of neurotensin receptors in various tissues and species.