The invention relates to mRNA therapy for the treatment of fibrosis and/or cardiovascular disease. mRNAs for use in the invention, when administered in vivo, encode human relaxin, isoforms thereof, functional fragments thereof, and fusion proteins comprising relaxin. mRNAs of the invention are preferably encapsulated in lipid nanoparticles (LNPs) to effect efficient delivery to cells and/or tissues in subjects, when administered thereto. mRNA therapies of the invention increase and/or restore deficient levels of relaxin expression and/or activity in subjects. mRNA therapies of the invention further decrease levels of toxic metabolites associated with deficient relaxin activity in subjects.