Abstract Introduction: The heptapeptide Angiotensin-(1–7) [(Ang-(1–7)] is a biologically active component of the Renin–Angiotensin System. Pharmacological studies often involve Ang-(1–7) radioactively labelled with 125I. Given the small size of the original peptide, we investigated whether introduction of a rather bulky iodine label interferes with the biological activity of Ang-(1–7). Methods: Ang-(1–7) was labelled with nonradioactive iodine with the chloramine-T method. The reaction products were separated on HPLC and analysed with mass spectrometry. The products were tested for biological activity in two ways: The ability of labelled Ang-(1–7) to block Ang II-induced contraction in rat aortic rings was tested in an organ bath setup. The affinity of labelled angiotensin for ACE in rat plasma was examined in vitro. Results: Iodination of Angiotensin-(1–7) resulted in two main products: monoiodinated and diiodinated Ang-(1–7) that could be easily separated on HPLC. In an organ bath experiment, monoiodinated Ang-(1–7) blocked Ang II responses identical to the native compound, whereas diiodinated Ang-(1–7) had lost its ability to block Ang II responses. Likewise, monoiodinated Ang-(1–7) had retained its affinity for ACE, while the affinity of diiodinated Ang-(1–7) was greatly reduced. Discussion: Monoiodinated Ang-(1–7) has a biological activity identical to the native compound, whereas this is lost in diiodinated Ang-(1–7). Therefore, only the monoiodinated radioactive form seems suited for pharmacological studies.