Receptor binding peptides labeled with medically important radionuclides such as technetium and rhenium are an important tool for the imaging and treatment of many forms of cancer. This paper describes a method of labeling peptides with rhenium using a natural amino acid chelating moiety. The structural characteristics of this chelate moiety, N-acetyl-cysteine-glycine-cysteine-glycine (NAc-CGCG) complexed with nonradioactive rhenium, have been investigated. The stability of this peptide-metal complex has been evaluated on the tracer level using radioactive rhenium-186. The rhenium-bound peptide has been appended to the N termini of receptor binding alpha-melanocyte stimulating hormone (alpha-MSH, NAc-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2) fragments via solid phase peptide synthesis. Bioassays and receptor binding studies of the resulting complexes demonstrate that the fragments retained biological activity and exhibited receptor binding constants ranging from 0.3 to 1.1 nM. This method could provide a general means of labeling bioactive peptide fragments that would simplify product purification and characterization.