An oligopeptide comprising pyroglutamyl, histidyl, tryptophyl, seryl, tyrosyl, D-leucyl, leucyl, arginyl, and N-ethylprolinamide residues joined in sequence. It is a synthetic nonapeptide analogue of gonadotropin-releasing hormone, and is used as a subcutaneous hydrogel implant (particularly as the acetate salt) for the treatment of prostate cancer and for the suppression of gonadal sex hormone production in children with central precocious puberty.