AbstractModified amino acids which can serve as precursors in the synthesis of tritium‐labelled peptides were prepared and incorporated into biologically active peptides. Thus,p‐iodophenylalanine was incorporated at position 4 of H‐Met(O2)‐Glu‐His‐Phe‐D‐Lys‐Phe‐OH (Org 2766) and in des‐Tyr1‐γ‐endorphin. As a second labelling site, the precursor of L‐leucine, L‐methallylglycine, was incorporated in both des‐Tyr1‐γ‐endorphin and the shorter des‐enkephalin‐γ‐endorphin. Finally, the lysine residue in Org 2766 (D‐Lys) and des‐enkephalin‐γ‐endorphin (L‐Lys) was replaced by its analogue 2,6‐diamino‐4‐hexynoic acid. The precursor peptides were synthesized via the fragment condensation approach. With the exception of the methallylglycine‐containing des‐Tyr1‐γ‐endorphin, in which partial desulfurization of the methionine residue could not be prevented, these peptides could be converted