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.