A linear (i.e., non-cyclic) analog of biologically active amphibian bombesin, mammalian gastrin-releasing peptide (GRP), or mammalian growth hormone releasing factor (GRF), having an active site and a binding site responsible for the binding of the peptide to a receptor on a target cell. Cleavage of a peptide bond in the active site of naturally occurring bombesin, GRP, or GRF is unnecessary for in vivo biological activity. The analog has one of the following modifications: (a) a deletion of an amino acid residue within the active site and a modification of an amino acid residue outside of the active site, (b) a replacement of two amino acid residues within the active site with a synthetic amino acid, a .beta.-amino acid, or a .gamma.-amino acid residue, or (c) a non-peptide bond instead of a peptide bond between an amino acid residue of the active site and an adjacent amino acid residue.