The present invention relates to a biologically active single chain Relaxin peptide having the following formula (I): Nter-X-(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-(X34)d-NH2-CterIt also concerns a pharmaceutical composition comprising at least one peptide of the invention, or a pharmaceutically acceptable salt or a solvate thereof, and the peptide, a pharmaceutically acceptable salt or solvate thereof, 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-X-(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-(X34)d-NH2-Cter。其亦關於包括至少一本發明胜肽,或其醫藥上可接受鹽或溶劑化物之醫藥組成物,以及此胜肽、其醫藥上可接受鹽或溶劑化物,或此醫藥組成物作為醫藥品之用途。