SUMMARYLysine8-vasqpressin was specif Scally labelled with tritium in the tysosine' and phenylalanine rgsidues. The 3,5-dibromo--tyrosine and 4-chloro-phenylalanine derivatives of the hormone were synthesized by solid phase peptide synthesis. The halogen containing peptides were catalytically reduced by tritium gas yielding preparations with high specific radioactivity and full biological activity as compared with reference material.