AbstractTyrosine derivatives were introduced onto the N‐terminal deprotected Pentagastrin to yield precursors of labelling. The tritiated derivative Boc‐3,5‐3H2‐Tyr‐Pentagastrin of high specific radioactivity (45 Ci/mmol) was obtained by catalytic dehalogenation of the corresponding di‐iodo‐derivative in the presence of tritium gas. Iodination of Boc‐Tyr‐Pentagastrin by NaI and chloramine T led to the formation of several compounds separated by HPLC. Conditions to obtain the mono‐iodinated non‐oxidized peptide, the only biologically active derivative, were investigated. Analytical chromatographic behaviour as well as biological activity of the tritiated and of the various iodinated p