The peptide encoded for by one of the two relaxin genes found in the human genome, designated H1, has been synthesized by the Boc-polystyrene solid phase method. The two chains which constitute relaxin, A- and B-, were assembled separately and, after cleavage, deprotection and purification, combined in solution at high pH to form the one intra- and two intermolecular disulfide bonds. Comprehensive chemical characterization including ion spray mass spectrometry of the peptide confirmed both its correct identity and high purity. The synthetic H1 relaxin was analyzed by circular dichroism spectroscopy and shown to possess a greater alpha-helical conformation in water than the corresponding H2 relaxin. The peptide had powerful direct chronotropic and inotropic effects in the isolated rat heart assay as did an analogue of the peptide in which the C-terminus of the B-chain was extended by four residues.