This invention provides a pharmaceutical composition comprising, as an active ingredient, a peptide derived from a PACAP or VIP peptide or a pharmaceutically acceptable salt thereof. This invention provides a PACAP/VIP derivative that can inhibit tautomerization in a solution and that can be applied to clinical applications over a long period of time. Such peptides are useful for ameliorating symptoms of diseases, such as regressive neurodegenerative diseases, erectile dysfunction, and bronchial asthma.