The present invention relates to a biologically active single chain Relaxin peptide having the following formula (I): Nter-Ac-(E)a-X10-E-G-R-E-X15-V-R-X18-X19-I-X21-X22-E-G-X25-S-X27-X28-X29-X30-R-(X32)b-(X33)c-(K)d-(X35)e-(gE)f-X37-Cter; or a salt or solvate thereof.It also concerns a pharmaceutical composition comprising at least one peptide of the invention, and the peptide or the pharmaceutical composition for its use as a medicament.本發明係關於具有下式(I)之生物活性單鏈鬆弛素胜肽:Nter-Ac-(E)a-X10-E-G-R-E-X15-V-R-X18-X19-I-X21-X22-E-G-X25-S-X27-X28-X29-X30-R-(X32)b-(X33)c-(K)d-(X35)e-(gE)f-X37-Cter;或其鹽或溶劑化物。其亦關於包括至少一本發明胜肽之醫藥組成物,以及其作為醫藥品用途之此胜肽或此醫藥組成物。