Metastin/kisspeptin is a 54 amino acid peptide ligand of the KISS1R receptor and is a critical regulator of GnRH secretion. The N-terminally truncated peptide, metastin(45-54), possesses a 10-fold higher receptor-binding affinity than full-length metastin and agonistic KISS1R activity but is rapidly inactivated in rodent plasma. We have developed a decapeptide analog [D-Tyr(45),D-Trp(47),azaGly(51),Arg(Me)(53)]metastin(45-54) with improved serum stability compared with metastin(45-54) but with decreased KISS1R agonistic activity. Amino acid replacements at positions 45-47 led to an enhancement of KISS1R agonistic activity and metabolic stability. N-terminal truncation resulted in a stable nonapeptide, [D-Tyr(46),D-Pya(4)(47),azaGly(51),Arg(Me)(53)]metastin(46-54), compound 26, which displayed KISS1R binding affinities comparable to metastin(45-54) and had improved serum stability. Compound 26 reduced plasma testosterone in male rats and is the first short-length metastin analog to possess testosterone suppressive activities. Compound 26 has led to the elucidation of investigational analogs TAK-683 and TAK-448, both of which have undergone clinical evaluation for hormone-dependent diseases such as prostate cancer.